Signalling Pathways in Apoptosis (Modern Genetics, Vol. 5)

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SIGNALLING PATHWAYS IN APOPTOSIS

Modern Genetics A series of books covering new developments across the entire field of genetics. Edited by Richard Lathe, Centre for Genome Research, University of Edinburgh, Kings Buildings, Edinburgh, EH9 3JQ, UK Volume 1 Embryonal Stem Cells: Introducing Planned Changes into the Animal Germline Martin L.Hooper Volume 2 Molecular Genetics of Inherited Eye Disorders edited by Alan F.Wright and Barrie Jay Volume 3 Molecular Genetics of Drug Resistance edited by John D.Hayes and C.Roland Wolf Volume 4 Animal Breeding: Technology for the 21st Century edited by A.John Clark Volume 5 Signalling Pathways in Apoptosis edited by Dianne Watters and Martin Lavin Additional volumes in preparation Genetics of Steroid Biosynthesis and Function edited by Ian Mason

This book is part of a series. The publisher will accept continuation orders which may be cancelled at any time and which provide for automatic billing and shipping of each title in the series upon publication. Please write for details.

SIGNALLING PATHWAYS IN APOPTOSIS Edited by

Dianne Watters and Martin Lavin The Queensland Institute of Medical Research Herston Australia

harwood academic publishers Australia • Canada • China • France • Germany • India • Japan Luxembourg • Malaysia • The Netherlands • Russia • Singapore Switzerland

This edition published in the Taylor & Francis e-Library, 2005. “To purchase your own copy of this or any of Taylor & Francis or Routledge’s collection of thousands of eBooks please go to www.eBookstore.tandf.co.uk.” Copyright © 1999 OPA (Overseas Publishers Association) N.V.Published by license under the Harwood Academic Publishers imprint, part of The Gordon and Breach Publishing Group. All rights reserved. No part of this book may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying and recording, or by any information storage or retrieval system, without permission in writing from the publisher. Amsteldijk 166 Ist Floor 1079 LH Amsterdam The Netherlands British Library Cataloguing in Publication Data Signalling pathways in apoptosis.—(Modern genetics; v. 5) 1. Apoptosis I. Watters, Diane, 1952–II. Lavin, Martin 571.9'36 ISBN 0-203-30365-2 Master e-book ISBN

ISBN 0-203-34392-1 (Adobe eReader Format) ISBN: 90-5702-392-X (Print Edition) ISSN: 1056-4497 (Print Edition) Front cover electron micrograph taken by Wen Yu and Deborah Stenzel of the Analytical Electron Microscope Facility, Queensland University of Technology, Gardens Point, Brisbane 4001.Australia.

CONTENTS

Preface to the Series

vii

Preface

viii

Contributors

ix

Overview: A Matter of Life and Death Gary Kasof, Kurt Degenhardt, Denise Perez, Anju Thomas and Eileen White

1

Part 1 Inducers of Apoptosis 1

The Death Receptors Klaus Schuhe-Osthoff and Marcus E.Peter

32

2

The Role of Sphingolipids in Stress Responses and Apoptosis in Eukaryotes Sheree D.Long and Yusuf A.Hannun

87

3

Radiation Response Pathways and Apoptosis Martin F.Lavin

105

Part 2 Regulation of Apoptosis 4

Kinase Cascades and Apoptosis John F.Hancock

125

5

Protein Kinase C Isoenzymes: Evidence for Selectivity in the Regulation of Apoptosis Janet M.Lord, Elizabeth M.Deacon, Lorna McMillan, Gareth Griffiths, Hema Chahal and Judit Pongracz

150

6

Apoptosis in Drosophila John M.Abrams, Po Chen, William Nordstrom and Johnson Varkey

177

7

Baculoviral Lessons in Apoptosis Christine J.Hawkins, Elizabeth J.Coulson and David L.Vaux

196

8

The Mitochondrion: Decisive for Cell Death Control? Catherine Brenner and Guido Kroemer

210

9

Caspases and the Commitment to Death Deborah M.Finucane, Thomas G.Cotter and Douglas R.Green

231

vi

Part 3 The Execution of Apoptosis 10

Caspases: The Molecular Effectors of Apoptosis Alison J.Butt and Sharad Kumar

251

11

Killer Cells—Deliverers of Exogenous Death Proteases Mark J.Smyth, Vivien R.Sutton and Joseph A.Trapani

268

12

Substrates of Cell Death Proteases and their Role in Apoptosis Dianne Watters and Nigel Waterhouse

298

Index

320

PREFACE TO THE SERIES

The Modern Genetics series, established under the editorship of Professor H. John Evans, was intended to cover new developments across the entire field of genetics of plants and animals, including man, and at all levels from the molecule to the population. This aim will be sustained and built upon, with increasing emphasis on the practical applications of the new genetics, be they in agriculture, medicine or biotechnology. The present volume underlines the contribution that molecular genetics has made to the understanding of basic cellular mechanisms. Indeed, the characteristic pattern of chromosomal fragmentation accompanying programmed cell death, and first described in 1980 by Andrew Wyllie here in Edinburgh, relied on the (then) relatively new technique of displaying DNA fragments by agarose gel electrophoresis. From that simple beginning has emerged the current concept that apoptosis is of fundamental importance to all aspects of development and differentiation. An ever-expanding field, the present state of the art is concisely compiled here by the editors of this volume, both acknowledged experts in the area. R.Lathe

PREFACE

In the five years since the publication of our first volume Programmed Cell Death: The cellular and molecular biology of apoptosis interest in apoptosis research has increased exponentially. At that time our understanding of the signalling pathways involved was in its infancy, some information was available on signalling through the Fas/Apol receptor but caspases had not yet come of age and the nuclease involved in DNA fragmentation remained elusive. It is now evident that there exist families of death receptors and their downstream effectors, the caspases, which cleave a set of crucial proteins with roles in cellular homeostasis, nuclear and cytoskeletal structure, and sensing and repairing DNA damage. While there are a large number of agents (radiation, chemotherapeutic agents, toxins and receptor ligation) which induce apoptosis by damaging or altering cellular functions in different ways, the central mechanisms remain highly conserved. In the case of death receptors the initial stimulus involves receptor ligation followed by recruitment of caspases and other signalling molecules prior to activation of a cascade of downstream caspases. These caspases cleave a number of molecules to activate them, including DNA fragmentation factor and/or I-CAD resulting in activation of a nuclease which fragments chromatin. Caspase cleavage also leads to inactivation of several proteins including DNA-PK, PARP and lamin which are involved in DNA damage repair and maintenance of nuclear structure respectively. The end result of this process is the packaging of cellular contents into apoptotic bodies in preparation for phagocytosis. While signalling through the death receptors has been relatively well described, the initiation of events resulting from cellular damage remains largely undefined. The aim of this volume is to describe the process of apoptosis at the induction, regulation, and execution phases. Emphasis is placed on the various death receptors, the pathways that are used either to induce or prevent apoptosis, and the steps involved in the cellular stress response. The pattern of activation of kinases that determines whether a cell will live or die is discussed. A greater understanding of the role of cytoplasmic events has been achieved by the identification of the mitochondrion as an apoptotic effector. Release of two apoptosis inducing factors (AIF and cytochrome c) from the mitochrondrion as well as the role of Bcl-2 in preventing this release are discussed. Convergence of the pathways activated by different agents appears to occur at the execution phase of apoptosis, where activated caspases cleave a series of molecules ultimately leading to the morphological changes characteristic of apoptosis. An understanding of all the intermediates involved in the process of apoptosis will provide a means of manipulating the system for the activation of apoptosis in cells resilient to death and the prevention of apoptosis in neurodegenerative and immune disorders, such as AIDS. While this treatment of apoptosis is not designed to be all embracing we trust that it will put into perspective the major events that occur in apoptosis from the initial stimulus to the execution process for at least some systems.

CONTRIBUTORS

Abrams, J.M. Department of Cell Biology and Neuroscience The University of Texas Southwestern Medical Center 5323 Harry Hinds Boulevard Dallas, TX 75235–9039 USA Brenner, C. Centre National de la Recherche Scientifique Unité Propre de Recherche 420 19 rue Guy Môquet F-94801 Villejuif France Butt, A.J. The Hanson Centre for Cancer Research Frome Road Adelaide, SA5000 Australia Chahal, H. Department of Immunology Birmingham University Medical School Birmingham, B152TT UK Chen, P. Department of Cell Biology and Neuroscience

x

The University of Texas Southwestern Medical Center 5323 Harry Hinds Boulevard Dallas, TX 75235–9039 USA Cotter, T.G. Tumor Biology Laboratory Biochemistry Department University College Cork Ireland Coulson, E.J.J. Walter and Eliza Hall Institute of Medical Research PO Royal Melbourne Hospital Parkville, VIC 3050 Australia Deacon, E.M. Department of Immunology Birmingham University Medical School Birmingham, B15 2TT UK Degenhardt, K. Center for Advanced Biotechnology and Medicine Rutgers University 679 Hoes Lane Piscataway, NJ 08854 USA Finucane, D.M. Division of Cellular Immunology La Jolla Institute for Allergy and Immunology 10355 Science Center Drive San Diego, CA 92121 USA Green, D.R. Division of Cellular Immunology La Jolla Institute for Allergy and Immunology 10355 Center Drive San Diego, CA 92121 USA Griffiths, G.

xi

Department of Immunology Birmingham University Medical School Birmingham, B15 2TT UK Hancock, J.F. Department of Pathology University of Queensland Medical School Herston Road Brisbane, QLD 4006 Australia Hannun, Y.A. Division of Oncology Department of Medicine Duke University Medical Center Durham, NC 27710 USA Hawkins, C.J. Division of Biology 156–29 California Institute of Technology 1201 East California Boulevard Pasadena, CA 91125 USA Kasof, G. Center for Advanced Biotechnology and Medicine Rutgers University 679 Hoes Lane Piscataway, NJ 08854 USA Kroemer, G. Centre National de la Recherche Scientifique Unité Propre de Recherche 420 19 rue Guy Môquet F-94801 Villejuif France Kumar, S. The Hanson Centre for Cancer Research Frome Road Adelaide, SA 5000 Australia

xii

Lavin, M.F. The Queensland Cancer Fund Research Unit The Queensland Institute of Medical Research PO Royal Brisbane Hospital Herston Brisbane, QLD 4029 Australia Long, S.D. Department of Medicine Duke University Medical Center Durham, NC 27710 USA Lord, J.M. Department of Immunology Birmingham University Medical School Birmingham, B15 2TT UK McMillan, L. Department of Immunology Birmingham University Medical School Birmingham, B15 2TT UK Nordstrom, W. Department of Cell Biology and Neuroscience The University of Texas Southwestern Medical Center 5323 Harry Hinds Boulevard Dallas, TX 75235–9039 USA Perez, D. Center for Advanced Biotechnology and Medicine Rutgers University 679 Hoes Lane Piscataway, NJ 08854 USA Peter, M.E. Tumor Immunology Program German Cancer Research Center

xiii

Heidelberg Germany Pongracz, J. Department of Immimology Birmingham University Medical School Birmingham, B15 2TT UK Schulze-Osthoff, K. Department of Internal Medicine I Medical Clinics Eberhard-Karls University Otfried-Müller Str. 10 D-72076 Tübingen Germany Smyth, M.J. Austin Research Institute Studley Road Heidelberg, VIC 3084 Australia Sutton, V.R. Austin Research Institute Studley Road Heidelberg, VIC 3084 Australia Thomas, A. Center for Advanced Biotechnology and Medicine Rutgers University 679 Hoes Lane Piscataway, NJ 08854 USA Trapani, J.A. Austin Research Institute Studley Road Heidelberg, VIC 3084 Australia Varkey, J. Department of Cell Biology and Neuroscience The University of Texas Southwestern Medical Center 5323 Harry Hinds Boulevard Dallas,TX 75235–9039

xiv

USA Vaux, D.L. Walter and Eliza Hall Institute of Medical Research PO Royal Melbourne Hospital Parkville, VIC 3050 Australia Waterhouse, N. The Queensland Cancer Fund Research Unit The Queensland Institute of Medical Research PO Royal Brisbane Hospital, Herston Brisbane, QLD 4029 Australia Watters, D. The Queensland Cancer Fund Research Unit The Queensland Institute of Medical Research PO Royal Brisbane Hospital, Herston Brisbane, QLD 4029 Australia White, E. Center for Advanced Biotechnology and Medicine Rutgers University 679 Hoes Lane Piscataway, NJ 08854 USA

OVERVIEW: A MATTER OF LIFE AND DEATH GARY KASOF*, KURT DEGENHARDT*, DENISE PEREZ*, ANJU THOMAS*, AND EILEEN WHITE*,**,† *Center for Advanced Biotechnology and Medicine **Department of Molecular Biology and Biochemistry, and the Cancer Institute of New Jersey; Rutgers University, Piscataway, New Jersey 08854

KEY WORDS: apoptosis, Bcl-2, capase, TNF-α, Fas, p53. Apoptosis or programmed cell death (PCD) is a genetically controlled process whereby cells die in response to environmental or developmental cues. The morphological characteristics of apoptosis include cytoplasmic blebbing, chromatin condensation and nucleosomal fragmentation (Wyllie, 1980). Dead cells are rapidly phagocytized to prevent damage to neighboring cells. Regulation of apoptosis is critical for normal development and tissue homeostasis and disruption of this process can have severe consequences (Jacobson et al., 1997). Too much cell death may produce neurodegenerative diseases and impaired development, while insufficient cell death can lead to increased susceptibility to cancer and sustained viral infection. Progress has been made in the past decade to identify many of the basic components that contribute to apoptosis, including transcriptional mediators, membranebound receptors (e.g. TNF-α receptor and Fas), Bcl-2 family members, kinases/ phosphatases, and cysteine proteases (White, 1996). Some of these proteins have been evolutionarily conserved from nematodes to mammals (Steller, 1995). bcl-2 was one of the first genes shown to regulate apoptosis (Hockenbery et al., 1990) and can inhibit apoptosis in a wide variety of systems (White, 1996). bcl-2 belongs to a growing family of genes that can either positively or negatively regulate apoptosis. One of these gene products, Bax, binds to Bcl-2 and antagonizes its ability to block apoptosis (Oltvai et al., 1993). Another critical element of the apoptotic process is the activation of cysteine proteases, which are currently referred to as caspases (Alnemri et al., 1996; Rao and White, 1997). In general, the caspases act downstream of Bcl-2-like proteins to induce apoptosis. Thus, the regulation of apoptosis appears to be a precarious balance between factors that promote survival and those responsible for initiating and executing cellular suicide. In this review, we present an overview of the basic components of apoptosis, like Bcl-2 and caspases, and how they are believed to function in this process.

† Corresponding Author: Center for Advanced Biotechnology and Medicine, 679 Hoes Lane, Piscataway, New Jersey 08854. Tel.: (732)235–5329. Fax: (732)235–5795. e-mail: [email protected]

2 G.KASOF, K.DEGENHARDT, D.PEREZ, A.THOMAS AND E.WHITE

TRANSCRIPTIONAL REGULATION OF APOPTOSIS Gene expression has been shown to be required for both apoptosis and survival depending on the cell type and stimulus. Indeed, inhibition of RNA and protein synthesis can block apoptosis induced by a number of circumstances, including glucocorticoid treatment (Cohen and Duke, 1983; Wyllie et al., 1984), growth factor deprivation (Martin et al., 1988; Scott and Davies, 1990), treatment with some chemotherapeutic drugs (Barry et al., 1990; Walker et al., 1991; Mizumoto et al., 1994), ischemia (Goto et al., 1990; Shigeno et al., 1990) and seizure (Schreiber et al., 1993). In contrast, others have shown that in some cases RNA and protein synthesis inhibitors either do not block or actually promote cellular demise (Rubin et al., 1988; Itoh et al., 1991; Gong et al., 1993; Vaux and Weissman, 1993). Some agents, such as the protein kinase inhibitor, staurosporine (Jacobson et al., 1994), and Fas (Schulze-Ostoff et al., 1994), can induce apoptotic-like events in enucleated cells. These data suggest that the basic machinery for apoptosis is constitutively expressed but can be modulated by changes in gene expression in order to trigger the death program. A number of transcription factors have been associated with induction of apoptosis, most notably p53; whereas others, such as NF-κB, have been implicated in promoting survival. p53 was first detected in rodent cells transformed by simian virus SV40 in a complex with the transforming protein SV40 T antigen, suggesting that it plays a role in growth control (Lane and Crawford, 1979; Linzer and Levine, 1979). Subsequently, it was noted that p53 mutations occur in a wide variety of tumors including lung, breast, colon, esophagus, liver, bladder, ovary, brain, and haemopoetic tissues (Hollstein et al., 1991; Levine et al., 1991). In fact, p53 loss-of-function mutations are the most common genetic alteration found in human cancer. Disruption of p53 in “knock out” mice results in spontaneous neoplasms by 6 months of age (Donehower et al., 1992). Furthermore, overexpression of wild-type p53 can suppress tumor formation in culture (Eliyahu et al., 1989; Finlay et al., 1989; Baker et al., 1990; Diller et al., 1990; Mercer et al., 1990). These data suggest that p53 functions as a tumor suppressor gene and has sparked extensive research into understanding its mechanism of action. p53 is a transcriptional regulatory protein capable of transactivating and repressing cellular genes (Ko and Prives, 1996; Levine, 1997). Transcriptional activation by p53 requires direct interaction with a sequence-specific DNA motif (PuPuPuC(A/T)(T/A) GPyPyPy) (El-Deiry et al., 1992; Farmer et al., 1992). p53-mediated transrepression, on the other hand, occurs on genes lacking the p53 DNA binding site and is probably dependent on interactions with components of the basal transcriptional machinery (Seto et al., 1992; Mack et al., 1993). Mutational analysis of p53 has revealed distinct domains that contribute to its gene regulatory activity (Prives, 1994). The N-terminus of p53 contains the activation domain, as well as binding sites for the cellular protein Mdm-2 (Momand et al., 1992) and the adenovirus E1B 55K protein (Kao et al., 1990). Complexes with both of these proteins interfere with p53 activity (Yew and Berk, 1992; Chen, Wu et al., 1996; Haupt et al., 1996). The central region of p53 has been shown by X-ray crystallography to contain sites for sequence-specific DNA binding (Cho et al., 1994). The C-terminus of p53 is critical for oligimerization (Stiirzbecher et al., 1992; Clore et al., 1994). Stable complexes of p53 with DNA suggest that p53 exists as a tetramer. A truncated protein containing just the Cterminal region can act as a dominant negative to inhibit p53 activity (Shaulian et al., 1992;

OVERVIEW: A MATTER OF LIFE AND DEATH 3

Eizenberg et al., 1996; Sabbatini et al., 1997). There also exists a proline-rich region between the activation and DNA binding domains that is thought to be capable of binding to SH3-containing proteins and could provide a link between p53 and signal transduction pathways (Walker and Levine, 1996; Sakamuro et al., 1997). The presence of these multiple domains underscores the functional, as well as the regulational, complexity of p53. Consistent with its role as a tumor suppressor gene, expression of p53 can induce either cell cycle arrest in G1 or apoptosis (Kastan et al., 1991; Yonish-Rouach et al., 1991). Functional p53 is required for apoptosis induced by ionizing radiation and chemotherapeutic drugs (Clarkè et al., 1993; Lowe et al., 1993), as well as transforming oncogenes like c-myc (Hermeking and Eick, 1994; Wagner et al, 1994; Sakamuro et al., 1995) and adenovirus E1A (Debbas and White, 1993). However, p53 is clearly not responsible for all modes of apoptosis, since its disruption does not appear to effect cell death triggered by glucocorticoids (Clarke et al., 1993; Lowe et al., 1993) or, for the most part, cell death observed during normal development (Donehower et al., 1992). The function of p53 is likely to depend on the cell type and/or physiological circumstances. Indeed, cytokines (Yonish-Rouach et al., 1991; Lin and Benchimol, 1995) and growth factors (Canman et al., 1995) can affect the ability of p53 to induce apoptosis or growth arrest. These results have led to the hypothesis that genotoxic induction of p53 acts as a checkpoint control to stop further progression in the cell cycle and thereby maintain genomic integrity (White, 1994). However, higher accumulation of p53, or induction accompanied by either a lack of growth factors/cytokines or the presence of a proliferative signal such as c-myc or E1A, results in cell suicide (Chen et al., 1996). p53-mediated growth arrest may at least in part be explained by its ability to transcriptionally induce the cdk inhibitor p21/WAF1/CIP1 (El-Deiry et al., 1993; Harper et al., 1993; Xiong et al., 1993; Brugarolas et al., 1995), as well as other possible growth inhibitory genes such as GADD45 (Kastan et al., 1992). The mechanism by which p53 induces apoptosis, however, is somewhat unclear. Generally, p53-mediated apoptosis is dependent on its gene regulatory capability. Point mutations in residues 22 and 23 of p53, which render p53 defective in both transcriptional activation and repression (Lin et al., 1994), block ElA-induced apoptosis (Sabbatini, Lin et al., 1995). However, Harris and colleagues have shown that the C-terminal region of p53, and not the transactivation domain, is required for apoptosis (Wang, Vermeulen et al., 1996). Another critical issue that remains is whether p53-dependent cell death involves activation or repression of cellular genes. Studies have indicated that at least in some cases p53-induced apoptosis is not affected by the presence of RNA and protein synthesis inhibitors, suggesting that repression rather than activation is the primary mechanism (Caelles et al., 1994). Several p53-repressible genes have been identified that could affect cell survival, including bcl-2 (Miyashita et al., 1994), MAP4 (Murphy et al., 1996), interleukin-6 (Santhanam et al., 1991), cyclin A (Desdouets et al., 1996), DNA topoisomerase IIα (Wang et al., 1997), c-fos (Kley et al., 1992), the TR2 steroid receptor (Lin and Chang, 1996), and c-myc (Moberg et al., 1992). The anti-apoptotic gene bcl-2 and its adenoviral homologue E1B 19K, which inhibit apoptosis (but not growth arrest) triggered by p53 (Debbas and White, 1993; Chiou, Rao et al., 1994), abrogate p53mediated repression suggesting one mechanism by which these genes function (Shen and Shenk, 1994; Sabbatini, Chiou et al., 1995). Alternatively, p53 can transactivate several genes that could contribute to apoptosis, including bax (Miyashita and Reed, 1995; Han,


Sabbatini, Perez et al., 1996), fas (Owen-Schaub et al., 1995), and insulin-like growth factorbinding protein-3 (IGF-BP3) (Buckbinder et al., 1995). Bax and Fas are bonified apoptotic inducers as will be discussed in the following sections. IGF-BP3, which is capable of inhibiting mitogenic signalling by the insulin-like growth factor IGF-1, could potentiate apoptosis via suppression of growth factor signalling. Thus, it is conceivable that both p53mediated repression and transactivation play a role in apoptosis. In addition to p53, several other transcription factors have been associated with apoptosis regulation, including Fos/Jun, c-Myc, Nur77 (NGFI-B), and NF-κB (Soares et al., 1994). The correlation between expression of these genes with cell death and survival suggests that they may be involved either directly, or indirectly, in the apoptotic program. For example, blocking Fos and Jun with antisense oligonucleotides inhibits apoptosis in lymphocytes following growth factor withdrawal (Colotta et al., 1992). Application of functional blocking antibodies against Fos and Jun (Estus et al., 1994), or dominant interfering Jun (Ham et al., 1995), inhibits apoptosis in neuronal cells following growth factor withdrawal. Overexpression of c-myc in myeloid cells (Askew et al., 1991) or fibroblasts (Evan et al., 1992) can also accelerate apoptosis, and antisense oligonucleotides against c-myc inhibit apoptosis in lymphocytes (Shi et al., 1992; Thulasi et al., 1993). Transdominant mutations of Nur77 (Woronicz et al., 1994), as well as antisense oligonucleotides (Liu et al., 1994), suppress apoptosis suggesting that this steroid receptor-like transcription factor may also have a positive role in cell death. Some transcription factors, like NF-κB, may actually protect cells from apoptosis. NF-κB-mediated inhibition of apoptosis plays a critical role in TNF-α signalling and will be discussed later in more detail (Beg and Baltimore, 1996; Van Antwerp et al., 1996; Wang, Mayo et al., 1996). These data suggest that cell death can be regulated by a number of transcriptional mediators which can either induce or suppress apoptosis. A critical issue that remains is to identify relevant downstream targets of these transcription factors. THE BCL-2 FAMILY One of the first mammalian genes discovered to regulate cell death was the anti-apoptotic gene bcl-2 (Vaux et al., 1988; Hockenbery et al., 1990). The role of bcl-2 as an inhibitor of apoptosis has since been established in many circumstances, including treatment with TNFα, Fas, UV radiation, chemotherapeutic drugs, growth factor/ hormone withdrawal, viral infection and tumor formation (White, 1996). The bcl-2 proto-oncogene was originally discovered as a common translocation in non-Hodgkins B-cell lymphoma (Bakhshi, 1985; Tsujimoto et al., 1985; Cleary et al., 1986). This chromosomal translocation event places the bcl-2 gene under the transcriptional control of the powerful enhancer elements of the immunoglobulin heavy chain resulting in high levels of Bcl-2 expression and the abrogation of normal programmed cell death and promotion of malignant potential. Disruption of bcl-2 in “knockout” mice leads to impaired kidney function manifested by polycystic disease and postnatal immune failure due to dramatic cell loss through apoptosis (Nakayama et al., 1993; Veis et al., 1993; Kamada et al., 1995).Thus,gain of Bcl-2 function is associ-ated with tumor development (Bakhshi, 1985; Tsujimoto et al., 1985; Cleary et al., 1986; McDonnell et al., 1989; McDonnell and Korsmeyer, 1991), while loss of Bcl-2 has only restricted consequences to normal development (Nakayama et al., 1993; Veis et al., 1993; Kamada et


Table 1 Bcl-2 family.

Bcl-2 homologues are catagorized according to apoptotic function. * Proteins which only contain BH3.

al., 1995). This suggests that there may be some redundancy in the Bcl-2 family or that other members have a more critical role (Table 1). One such gene, bcl-xL, encodes a death repressing protein that may play a more general role in the regulation of apoptosis since it is widely expressed during mouse development (Boise et al., 1993). Bcl-xL is one of two products of the bcl-x gene (Boise et al., 1993). A shorter spliced variant of bcl-x, bcl-xS, is an inducer of apoptosis and antagonizes the protective activity of bcl-xL (Boise et al., 1993). In contrast to bcl-2, disruption of the bcl-x gene leads to embryonic lethality with massive cell death in haemopoetic cells as well as the developing nervous system (Motoyama et al., 1995). In addition to bcl-xL, other bcl/-2-like genes exist (e.g. Mcl-1, Bfl-1, Bcl-w, and A1), some of which may be tissue specific and thereby participate in unique modes of apoptosis (Table 1). Mcl-1 is an apoptotic inhibitor whose expression increases early in the differentiation of a myeloblastic leukemia cell line (Reynolds et al., 1994). Expression of another Bcl-2 homologue, Bfl-1, is correlated with development of stomach cancer (Choi et al., 1995). The gene bcl-w was identi-fied in both murine peripheral blood cells and brain cDNA libraries as an inhibitor of apoptosis in FDCP1 cells following interleukin-3 withdrawal (Gibson et al., 1996). Human endothelial cells, which have low levels of Bcl-2, express a homologue, A1, that is induced in response to TNF-α treatment (Karsan et al., 1996). Thus, inhibition of apoptosis is likely to be mediated by a complex set of Bcl-2-like proteins. It is now realized that bcl-2 has also been evolutionarily conserved (Table 1). Homologues exist in the nematode C.elegans (ced-9 (Hengartner et al., 1992)) as well as many viruses (adenovirus, E1B 19K (Rao et al., 1992; White et al., 1992; Boyd et al., 1994; Chiou,Tseng et al., 1994); Epstein-Barr virus, BHRF1 (Henderson et al., 1993); African swine fever virus, LMW5-HL (Neilan et al., 1993); human herpesvirus 8, KSbcl-2 (Cheng et al., 1997); and herpesvirus saimiri, ORF16 (Nava et al., 1997)). Like bcl-2, these worm and viral counterparts can inhibit apoptosis. These findings suggest that the apoptotic program has been at least partly conserved throughout evolution.


Several bcl-2-like genes have been identified that can promote, rather than inhibit, apoptosis. The best characterized and prototypic gene of this class is bax (Oltvai et al., 1993). While overexpression of bax can induce apoptosis, disruption of this gene in “knockout” mice leads to lymphoid hyperplasia (Knudson et al., 1995). Recent data also demonstrates that expression of Bax can suppress tumorigenesis in vivo (Yin et al., 1997). These data suggest that Bax plays a critical role during apoptosis. Biochemical studies have indicated that Bax directly interacts with itself as well as several anti-apoptotic proteins such as Bcl-2, Bcl-xL, and E1B 19K (Oltvai et al., 1993; Sedlak et al., 1995; Han, Sabbatini, Perez et al., 1996). Interaction with these inhibitors of cell death, functionally antagonize the death-promoting activity of Bax. Other bcl-2 homologues have also been identified which may function similarly to bax, including bad and bak (Table 1). The death-promoting gene, bad, was isolated from a yeast two-hybrid and expression cloning screens against bcl-2 (Yang et al., 1995). Bad strongly interacts with Bcl-xL and reverses its suppression of apoptosis (Yang et al., 1995). Another gene, bak, was identified simultaneously by a twohybrid screen as well as a PCR based strategy aimed at looking for bcl-2-related genes(Chittenden et al., 1995; Farrow et al., 1995; Kiefer et al., 1995). Bak interacts with Bcl-xL and EIB 19K and in most cases accelerates cell death. Interaction between death-promoting and suppressing Bcl-2-like proteins has led to a rheostat model for explaining how these proteins function (Oltavi and Korsmeyer, 1994). According to this model, the ratio between survival factors, such as Bcl-2, and death promoters, such as Bax, controls the fate of the cell. However, the biochemical mechanism of these proteins is still unclear. What is also unclear is which set of bcl-2 proteins act as effector molecules. For example, do the death promoting genes simply inhibit a required survival factor (e.g. Bcl-2, Bcl-xL) or do they actually trigger apoptosis? Bcl-2 and its family members bind to a number of unrelated proteins that has provided insight into their apoptotic activity. For example, Bcl-2 interacts with the small molecular weight G-protein, R-ras (Fernandez-Sarabia and Bischoff, 1994), and expression of R-ras can antagonize Bcl-2 function (Wang, Milan et al., 1995). Although related proteins, such as H-ras, N-ras, and K-ras, contribute to oncogenic transformation, R-ras does not exhibit this property. However, R-ras can promote apoptosis following growth factor deprivation (Wang, Milan et al., 1995). The role of R-ras in signal transduction is unclear although it is known that it interacts with the serine/threonine kinase Raf-1 (Spaargaren et al., 1994). Bcl-2 cooperates with Raf-1 to inhibit apoptosis and targets Raf-1 to the mitochondrial membrane (Wang, Rapp et al., 1996). Interestingly, other Bcl-2 binding proteins can regulate Raf-1 activity. Bag-1, a protein that interacts and cooperates another Bcl-2 family member with Bcl-2, also binds to Raf-1 (Takayama et al., 1995). Serine phosphorylation of Bad results in binding to the protein, 14–3–3, which is an activator of Raf-1 (Li et al., 1995; Zha, Harada et al., 1996). Phosporylation of Bad is triggered by the kinase Akt in response to cytokines and inhibits Bad-induced cell death (Datta et al., 1997; del Peso et al., 1997). Thus, the Bcl-2 family and associated proteins R-ras, Bag-1, and 14–3–3 may function by regulating the activity of kinases and in turn may be regulated by kinases themselves. E1B 19K, but not Bcl-2, interacts with the nuclear structural protein, lamin A/C (White et al., 1984; White and Cipriani, 1989; Rao et al., 1996; Rao et al., 1997). The lamins are cleaved by caspases during apoptosis and lamin proteolysis is required for the nuclear events of apoptosis (Lazebnik et al., 1995; Rao et al., 1996; Takahashi et al., 1996). Binding to


lamin A/C, however, does not prevent lamin cleavage (Rao et al., 1997). Rather, lamin A/ C may serve to target E1B 19K to the nuclear envelope. 53Bp2 was identified in three separate two-hybrid screens using Bcl-2, p53, and phosphorylase phosphatase-1 (PP-1) as baits (Helps et al., 1995; Gorina and Pavletich, 1996; Naumovski and Cleary, 1996). 53Bp2 inhibits p53 DNA-binding and induces G2/M growth arrest (Naumovski and Cleary, 1996). Phosphorylation of p53, such as by cyclin-dependent kinases, is associated with p53 transcriptional activity (Wang and Prives, 1995). This supports a model whereby 53Bp2 binding to a phosphatase may inhibit p53 activity, thereby abrogating p53-mediated apoptosis and inducing G2/M arrest. It is intriguing that Bcl-2 can also bind to 53Bp2, suggesting that the Bcl-2 family may also have a role in controlling p53 function. Recent structural experiments on Bcl-2-like proteins have also helped to elucidate their biochemical mechanism. The Bcl-2 family members contain four conserved regions (BH1, BH2, BH3, and BH4) that are important for protein-protein interactions and apoptotic regulation (Reed et al., 1996). Point mutations in Bcl-2 within the BH1 (gly145) and BH2 (trp188) domains eliminate its association with Bax and compromise its death repressing activity but has no effect on homodimerization (Yin et al., 1994). In contrast, a deletion mutant of Bcl-2 which lacks the BH4 domain is unable to block apoptosis and homodimerization, but is still capable of binding to Bax (Reed et al., 1996). Thus far, the BH4 domain has only been found in the survival-promoting proteins. The BH3 domain was originally identified as a region within Bax that is essential for homodimeration as well as interaction with antagonist proteins such as E1B 19K and Bcl-2 (Han, Sabbatini, Perez et al., 1996; Zha, Aimé-Sempé et al., 1996). A chimeric protein of Bcl-2 containing the BH3 domain of Bax provides death promoting activity to Bcl-2 (Hunter and Parslow, 1996). Interestingly, a number of proteins exist that contain just the BH3 domain, including Nbk/Bik (Boyd et al., 1995; Han, Sabbatini and White, 1996), Hrk (Inohara et al., 1997), Bid (Wang, Yin et al., 1996), and Bcl-xS (Boise et al., 1993) (Table 1). These proteins can bind to and antagonize Bcl-2-like survival proteins. While these data suggest that the BH3 domain plays a more critical role in triggering apoptosis, some Bcl-2 like proteins such as Bad can induce cell death but do not a contain a BH3 domain (Yang et al., 1995). Thus, it is likely that while these conserved regions have some role in protein function, their exact significance may be protein dependent and will probably require a further understanding of their structure. Recently, the structure of Bcl-xL was solved using NMR and X-ray crystallographic techniques (Muchmore et al., 1996). Bcl-xL contains 5 amphipathic α-helices surrounding 2 central hydrophobic helices. Although spread throughout the primary amino acid sequence, the functionally important BH1, BH2, and BH3 domains are in close spatial proximity forming a hydrophobic cleft that is involved in mediating protein-protein interactions important for its anti-apoptotic function (Sattler et al., 1997). An interesting observation from these studies is that the structure of Bcl-xL resembles the membrane insertion domain found in bacterial toxins such as colicins and diphtheria toxin. These proteins insert into cellular membranes and multimerize to form pores (London, 1992), suggesting that Bcl-xL may regulate apoptosis through a pore-forming activity. Indeed, Bcl-2 related proteins are generally found associated with membrane structures, particularly the mitochondria, endoplasmic reticulum, and the nuclear envelope (White et al., 1984; Monagan et al., 1992;


Gonzalez-Garcia et al., 1994). Furthermore, in vitro experiments demonstrate that Bcl-xL can facilitate ion transport across a lipid bilayer (Minn et al., 1997). It will be of interest to determine if the Bcl-2 family members actually function as channels in vivo and if this activity is responsible for apoptosis regulation. These and other studies have led to several hypotheses regarding the biochemical mechanism of this family of proteins. One possibility that has emerged is that they form a channel capable of regulating ion flux, such as calcium. Calcium is a critical component of signalling pathways and high levels of intracellular calcium is often associated with cell death (Trump and Berezesky, 1995). Bcl-2 can suppress calcium release from the endoplasmic reticulum, although it is possible that this is simply a downstream consequence of its ability to inhibit apoptosis (Lam et al., 1994). Nevertheless, the experiments indicating that the Bcl-2-like proteins can form ion channels adds some credence to this model. Others have speculated that Bcl-2 regulates the generation of reactive oxygen radicals since Bcl-2 is able to attenuate cell death induced by oxidative damaging agents (Hockenbery et al., 1993; Zhong et al., 1993; Satoh et al., 1996). Free radicals can contribute to cell death by disrupting DNA, proteins, and lipids (Olanow, 1993). The localization of Bcl-2 at mitochondrial membranes makes it plausible that the protein is directly involved in oxidative phosphorylation (Hockenbery et al., 1990). However, further evidence demonstrates that Bcl-2 can inhibit apoptosis in cells that lack mitochondrial DNA and thereby respiration (Jacobson et al., 1993). An alternative mechanism for Bcl-2 action at the mitochondria may be the control of cytochrome c release. Release of cytochrome c from the mitochondria into the cytosol is required for caspase activation and DNA fragmentation in cell-free extracts (Liu, Kim et al., 1996), and Bcl-2 can act to suppress cytochrome c efflux from mitochondria(Kluck et al., 1997; Yang et al., 1997). It will certainly be important to determine if efflux of proteins, such as cytochrome c, from the mitochondria actually contributes to apoptosis in vivo or if this is simply a downstream phenomenon. Additionally, there is tremendous interest in elucidating the downstream events of apoptosis, most notably, caspase activation. CASPASES Evidence suggesting that caspases play an integral part in the final executionary steps of the apoptotic pathway was first elucidated with the discovery that the ced-3 gene product, which is required for programmed cell death in the nematode C. elegans, has a high sequence homology to ICE (caspase-1) (Yuan et al., 1993). Caspase-1, is a mammalian cysteine protease that specifically cleaves pro-IL-1β into the mature IL-1β cytokine in response to inflammation (Thornberry et al., 1992). Activation of caspase-1 or related proteases has been shown to be a common downstream component of the apoptotic program (Rao and White, 1997). In general, nascent caspases consist of a regulatory prodomain followed by two domains that are cleaved into two subunits which bind to form a heterotetrameric active protease. ICE consists of a long N-terminal prodomain that is cleaved at a conserved aspartic acid residue as one of the steps in protease activation. The C-terminal portion is cleaved after a putative aspartic acid residue within the consensus pentapeptide sequence QACRG to produce two subunits (p10 and p20). The 20 kDa catalytic subunit contains a conserved cysteine residue that is important for substrate recognition and the 10 kDa


subunit confers substrate specificity. The subunits are then capable of interacting with one another to form an active tetrameric protease (p20:p10)2 that is able to bind and cleave substrates for subsequent steps of apoptosis. Thus far, 10 additional mammalian proteases have been discovered that are homologous to caspase-1 (Table 2). Based upon sequence homology, these proteases have been classified into three major categories, ICE, CPP32 (Ced-3-like), or Ich-1 (Anemri et al., 1996). The role of these proteases in programmed cell death has been aided by the identification of specific caspase inhibitors, such as CrmA. CrmA, a gene product of cowpox virus, acts as a strong competitive inhibitor of caspase-1 (Ray et al., 1992; Miura et al., 1993; Komiyama et al., 1994) as well as a weak inhibitor of caspase-3 (Nicholson et al., 1995), and blocks numerous apoptotic signals, including TNF-α, Fas, and serum withdrawal (Enari et al., 1995; Los et al., 1995; Tewari and Dixit, 1995). Similarly, p35, a protein product of the baculovirus, acts as an irreversible inhibitor by blocking autoactivation of caspase-1 -2, -3, -4 and Ced-3 (Bump et al., 1995; Xue and Horvitz, 1995). Synthetic chemical inhibitors have recently been developed that mimic the substrate pentapeptide cleavage sequence with the residue at the P4 position determining specificity (Nicholson, 1996; Rotonda et al., 1996). The specific caspase-1 inhibitor acetyl-Tyr-Val-Ala-Asp-chloromethylketone (AcY-VADCHO) has a hydrophobic residue at the P4 position that confers specificity within the protease’s binding pocket. Acetyl-Asp-Glu-Val-Asp-chloromethylketone (Ac-DEVDCHO), however, is a potent inhibitor of caspase-3 and other Ced3-like proteases due to the aspartic acid residue at the P4 position. These synthetic inhibitors allow one to differentiate between the different classes of proteases in kinetic inhibitory studies and thereby develop a protease activation cascade model. Application of either synthetic inhibitors or CrmA blocks Fas-and TNF-α-induced apoptosis. However, the caspase-1 specific inhibitor Ac-Y-VADCHO blocks caspase1-like and caspase-3-like activation as opposed to Ac-DEVD-CHO which only prevents activation of caspase-3-like proteases (Enari et al., 1996). Thus, one can conclude that a sequential cascade-like activation of both proteases is involved in mediating this apoptotic pathway. Caspase-3, one of the most highly studied downstream proteases, is activated by several proteases including caspases-1, -6, -8, and -10 (Tewari et al., 1995; Fernandes-Alnemri et al., 1996; Srinivasula et al., 1996), as well as the serine protease granzyme B (Darmon et al., 1995). Its activation is inhibited by CrmA (Nicholson et al., 1995; Tewari et al., 1995) and also more potently by Z-VAD, DEVD-CHO, p35 (Bump et al., 1995), and E1B 19K and Bcl-2 (Boulakia et al., 1996; Sabbatini et al., 1997) in several apoptotic pathways and in various cell types. In contrast to caspase-1 knockout mice whose phenotype is relatively normal (although they do have defects in their inflammatory response), caspase-3 null mice die at 1–3 weeks of age, and show brain and neuronal defects early in embryonic development with fewer pyknotic clusters indicative of a decreased rate of apoptosis (Kuida et al., 1996). Fewer thymocytes are also present but are able to undergo apoptosis by Fas antibody, dexamethasone and staurosporine, possibly due to the presence of redundant proteases such as caspases-6 or -7. Several groups are currently seeking the downstream targets of caspases that contribute to apoptosis. Among the first substrates identified was poly(ADP-ribose) polymerase (PARP), a DNA damage sensing and repair protein that is cleaved to an 85 kDa protein very early in the apoptotic process (Kaufmann et al., 1993). This protein is also one of the components


Table 2 Human caspase family.

cleaved by an ICE-like enzyme in a cell free system (Lazebnik et al., 1994). PARP, however, appears not to be essential in the apoptotic process since mice containing a mutation in (ADP-ribosyl) transferase (ADPRT), which compromises poly(ADP-ribosyl)ation, are able to develop relatively normally (Wang, Auer et al., 1995). Several other proteins are cleaved by caspases and their role in apoptosis is being evaluated. Lamins A/C and B are substrates for proteases in a cell free system with lamin A/ C being specifically cleaved by caspase-6 but not caspase-3 or -9 (Lazebnik et al., 1995; Orth et al., 1996;Takahashi et al., 1996). Y-VAD-cmk and TLCK inhibits lamin cleavage but not PARP cleavage (Lazebnik et al., 1995). Thus, one can conclude there are multiple proteases that cleave specific substrates. The role of lamins as relevant substrates in apoptosis has been determined through studies demonstrating that cells that express mutant lamins with an altered caspase cleavage site have attenuated onset of apoptosis, perhaps by maintaining the nuclear structural integrity (Rao et al., 1996). The disassembly of the structural components of the cell may facilitate cell death. Another protein, DNA fragmentation factor (DFF), is cleaved by caspase-3 generating an active protein capable of inducing DNA fragmentation (Liu et al., 1997). Other possible protease targets include Rb (Bing and Dou, 1996), SREBP, U1–


70 Kd ribonucleoprotein, Gas 2, DNA depend-ent kinases and actin, but the relevance of the cleavage of these proteins by caspases in the apoptotic pathway remains to be determined (Fraser and Evan, 1996). Caspases are an evolutionarily conserved family of proteins that not only execute the final steps of apoptosis but also, in some cases, regulate upstream induction of cell death. There appears to be a family of regulatory proteases upstream in the protease cascade that sequentially activates a second set of proteases. Lengthy prodomain proteases tend to be activated upstream in the apoptotic process by interacting directly with the receptor complex (or possibly other apoptotic genes), and then these caspases activate a second set of smaller prodomain proteases which contribute to the final steps of apoptosis. Evidence suggests these secondary proteases cleave the structural substrates that are necessary for the morphological changes associated with apoptosis. Within this cascade of events there may be redundancy amongst the proteases. Thus, determining which proteases are pertinent for each cell type and apoptotic pathway and the substrates they cleave remains to be elucidated. Much effort has been made recently to elucidate the mechanism by which the Bcl-2 family regulates caspases (Rao and White, 1997). In general, it appears that the Bcl-2-like proteins are upstream of the cysteine proteases in the apoptotic pathway. Thus, inhibition of cell death by Bcl-2, EIB 19K, or Bcl-xL abrogates activation of the known proteases (Boulakia et al., 1996; Chinnaiyan, Orth et al., 1996). A significant development was achieved by the discovery in C. elegans that Ced-4 acts as a bridge to join Ced-9 (Bcl-2-like) and Ced-3 (caspase) (Chinnaiyan et al., 1997; Spector et al., 1997; Wu et al., 1997). An analogous scenario may be present in mammals. Indeed, Ced-4 binds to Bcl-xL as well as the prodomains of caspase-1 and -8 (Chinnaiyan et al., 1997; Wu et al., 1997). In addition, a mammalian protein, Apaf-1, has recently been identified that contains homology to Ced-4 (Zou et al., 1997). Thus far, Apaf-1 has only been reported to interact with Bcl-xL, although future studies may reveal associations with other Bcl-2 family connection between the Bcl-2 family and caspase activation. THE TNF-A/FAS PATHWAY OF APOPTOSIS TNF-α and FasL, as previously mentioned, are potent cytokines that are capable of inducing cell death by apoptosis (Tartaglia et al., 1993; Nagata, 1997). TNF-α is secreted predominantly by activated macrophages and has pleiotropic activities including cytotoxicity during inflammation, immunoregulation, and antiviral responses (Beutler and Cerami, 1986). Signalling by TNF-α is mediated by two distinct and widely expressed receptor subtypes, TNFR1 (55 kDa) and TNFR2 (75 kDa) (Tartaglia et al., 1991). In most cases TNFR1 is responsible for TNF-α-induced apoptosis and activation of the transcription factor NF-κB (Tartaglia et al., 1991). Signalling through TNFR2 occurs less frequently and appears to be restricted to cells of the immune system. FasL binding to its receptor Fas (also known as CD95 and Apo-1) induces apoptosis and plays a critical role in apoptosis triggered by activated T and B cells (Singer and Abbas, 1994; Lagresle et al., 1995; Rothstein et al., 1995) and in maintaining immune privilege sites (Griffith et al., 1995). The expression of Fas and FasL is greatly reduced in lpr (lymphoproliferation) and gld (general lymphoproliferative disease) mutant mice, respectively (WatanabeFukunaga et al., 1992;


Takahashi et al., 1994). Lpr and gld mice develop autoimmune diseases as well as lymphadenopathy and splenomegaly by accumulating CD4-CD8-thymocytes. Humans with mutations in the Fas pathway, develop autoimmune disorders (Fisher et al., 1995; RieuxLaucat et al., 1995; Giordano et al., 1997). A new mechanism has recently been proposed for cells of the immune privilege sites such as the eye and the testes to evade inflammatory responses by activated inflammatory cells (Bellgrau et al., 1995; Griffith et al., 1995). Activated immune cells expressing Fas enter the immune privilege sites but are thought to be rapidly killed by FasL expressed by cells in these sites. It has also been demonstrated that cancer cells express FasL to kill immune effector cells in order to evade the immune system (Hahne et al., 1996). Infiltrating T cells expressing Fas were rapidly killed by melanoma cells expressing FasL in vitro. In contrast, Fas-deficient lpr mutant mice immune effector cells were not killed by FasL expressing cells. Thus, the phenotypes in mice and humans suggest that Fas induces signals responsible for cell death and deletion of lymphocytes as well as other cell types. The biochemical mechanism of TNF-α and FasL signalling have been well studied and it has been noted that they share many similarities (Nagata, 1997). The receptors for both of these cytokines, TNFR1/2 and Fas, are members of a growing receptor family that includes CD27, CD30, CD40, OX40, 4-1BB, lymphotoxin-β receptor, NGF receptor (Smith et al., 1994), DR-3/Wsl-l (Chinnaiyan, O’Rourke et al., 1996; Kitson et al., 1996), CARl (Brojatsch et al., 1996),and DR-4(Pan et al., 1997). This family is defined by an extracellular group of cysteine-rich repeats, and they generally play important roles in regulating cell survival. In addition, Fas and TNFR1 both contain a death domain in the cytoplasmic region of about 80 amino acids, that is required for initiation of cell death (Itoh et al., 1991; Tartaglia et al., 1993). Death domains are also present in other family members including DR-3/Wsl-l, DR-4 and CAR1 (Brojatsch et al., 1996; Chinnaiyan, O’Rourke et al., 1996; Kitson et al., 1996; Pan et al., 1997). Overexpression of these receptors has been associated with apoptosis. Deletion of the death domain in either TNFR1 (Tartaglia et al., 1993), Fas (Cascino et al., 1996), DR-3/Wsl-l (Chinnaiyan, O’Rourke et al., 1996; Kitson et al., 1996), or DR-4 (Pan et al., 1997) abrogates their apoptotic response. NMR and mutational analysis of the Fas death domain has revealed that it consists of six antiparallel, amphipathic α-helices and that there are specific regions that allow for self association as well as binding to downstream partners (Huang et al., 1996). It is therefore hypothesized that the mechanism of these receptor molecules is to recruit other death-associated adaptor proteins via death domain interactions. A novel 34 kDa protein termed TNF-associated death domain protein (TRADD) was identified by specific interaction with the death domain of TNFR1 using the yeast twohybrid system (Hsu et al., 1995). Overexpression of TRADD in a wide variety of cell lines can trigger apoptosis (Hsu et al., 1995). Interestingly, TRADD does not interact with Fas even though Fas and TNFR1 contain homologous death domains in their intracellular region (Hsu et al., 1995). However, this disparity between TRADD recruitment to TNFR1 but not Fas has been resolved by the identification of another death domain protein, Fas-associated death domain protein (FADD/MORT 1), which binds TRADD as well as the cytoplasmic region of Fas (Boldin et al., 1995; Chinnaiyan et al., 1995; Chinnaiyan, Tepper et al., 1996). Overexpression of FADD can also induce apoptosis in several cell types (Boldin et al., 1995; Chinnaiyan et al., 1995). The C-terminal region of FADD contains a death domain


homologous to the cytoplasmic regions of Fas and TNFR1 (Chinnaiyan, Tepper et al., 1996). The death domain allows interaction of FADD with Fas and TRADD, but not with TNFR1 (Chinnaiyan et al., 1996). The N-terminal portion of FADD contains a death effector domain (DED) that is responsible for binding downstream effector proteins (Chinnaiyan, Tepper et al., 1996). Deletion mutants of FADD which lack this N-terminal region, act in a dominant negative fashion to block TNF-α- and FasL-induced apoptosis, suggesting that effector molecule interactions are required for cell death signalling (Chinnaiyan, Tepper et al., 1996). Recently, an effector protein was identified termed FLICE/MACH (caspase-8) that is capable of binding to FADD in the yeast two-hybrid assay and in vivo and can induce apoptosis when overexpressed (Boldin et al., 1996; Muzio et al., 1996). The N-terminal prodomain of caspase-8 contains two DEDs of approximately 60 amino acids that is homologous to the DED of FADD, while the C-terminal region of caspase-8 shows homology with the ICE family of proteases (Boldin et al., 1996; Muzio et al., 1996). Caspase-8 directly interacts with the DED of FADD leading to activation of the protease and apoptosis (Boldin et al., 1996; Muzio et al., 1996). Indeed, TNF-α-and Fas-mediated apoptosis are both inhibited by expression of the caspase inhibitor CrmA (Enari et al., 1995; Los et al., 1995; Tewari and Dixit, 1995; Muzio et al., 1996). This has led to the following model for TNF-α and FasLinduced cell death (Figure 1). Ligand binding leads to oligimerization of the respective receptors and facilitates binding of TRADD and FADD. In TNF-α-induced apoptosis, TRADD then binds FADD so that there is recruitment of FADD in both pathways. FADD subsequently binds to and recruits caspase-8 to the DISC allowing the caspase to be activated. Caspase-8 is capable of autocatalyzing the cleavage of its prodomain releasing the active C-terminal subunits. This is believed to begin a cascade of protease activation leading toward cell death (Srinivasula et al., 1996). Multimerized FADD produces filaments throughout the cell (Perez and White, 1998; Siegel et al., 1998). E1B 19K, which blocks FADD-induced caspase activation and cell death, may work in part by disrupting FADD filaments causing FADD to relocalize with regions normally associated with 19K (Perez and White, 1998). It will be interesting to determine whether other Bcl-2 family members have a similar function. In addition to FADD, a number of other Fas-interacting proteins have been identified by two hybrid screening using the cytoplasmic region of Fas as the bait. One of these proteins, RIP, contains a C-terminal death domain as well as an N-terminal region that is strongly homologous to serine/threonine kinases (Stanger et al., 1995). Transient expression of RIP results in both apoptosis and NF-κB activation, although the mechanism for these activities appear to be distinct (Stanger et al., 1995; Ting et al., 1996). For example, deletion of the death domain of RIP abrogates its apoptotic ability but does not effect its capacity to activate NF-κB (Stanger et al., 1995; Liu, Hsu et al., 1996). Death domains may play a role in binding to another death domain containing protein, RAIDD, which is capable of recruiting the cysteine protease caspase-2 and therefore may initiate a protease cascade similar to FLICE (Duan and Dixit, 1997). Another novel Fas binding protein, FAF-1 (Fas-associated protein factor) can also induce apoptosis following transient transfection (Chu et al., 1995). FAP-1 (Fas-associated protein), is a tyrosine protein phosphatase that appears to suppress cell death (Sato et al., 1995). Overexpression of FAP-1 in a T cell line attenuates Fasinduced apoptosis (Sato et al., 1995). FAP-1 expression is highest in cell lines resistant to


Figure 1 TNF-α and Fas signalling pathways.

Fas-induced apoptosis (Sato et al., 1995). The regulation of apoptosis by FAF-1 and FAP-1 are thus far unknown. TNF-α and Fas signalling also promotes a kinase cascade leading to the activation of the stress-responsive, mitogen-activated kinases, p38 (MAP kinase) and JNK (Jun kinase) which have been implicated as potential mediators of apoptosis (Liu, Hsu et al., 1996; Juo et al., 1997; Nishina et al., 1997). These kinases trigger changes in transcriptional regulation particularly by AP-1 which have been reported to play a role in numerous physiological processes including cell death (Soares et al., 1994). The role of these kinases in TNF-α and Fas mediated apoptosis is poorly understood. The MAP kinase, ASK1, is activated upon TNF-α treatment (Ichijo et al., 1997). Overexpression of ASK1 induces apoptosis and a catalytic inactive form of ASK1 blocks TNF-α-induced cell death (Ichijo et al., 1997). ASK1 phosphorylates other MAP kinases such as SEKl and MKK3/ MAPKK6 which in turn can activate the JNK and p38 respectively, suggesting that the MAP kinases can trigger apoptosis (Ichijo et al., 1997). However, others have suggested that the stress kinase pathway does not contribute to cell death induced by TNF-α and Fas (Liu, Hsu et al., 1996; Lenczowski et al., 1997). Expression of a deletion mutant of RIP lacking the death domain does not induce apoptosis but is still able to activate JNK (Liu, Hsu et al., 1996). Furthermore, the specific p38 inhibitor SB 203580 completely blocks Fas-induced p38 activation but has no effect on inhibiting cell death (Stanger et al., 1995; Liu, Hsu et al., 1996). Disruption of SEK1, a direct activator of JNK, enhanced susceptibility to Fas-induced apoptosis, suggesting that the kinase pathway may have a protective role (Nishina et al., 1997). Thus, it appears that the kinase cascade initiated by TNF-α and Fas may have a role in either inducing or protecting


from cell death; however, the mechanism and significance of this process remains controversial. In addition to the apoptotic response elicited by TNF-α, the cytokine also induces a distinct pathway leading to the activation of the transcription factor NF-κB (Tartaglia et al., 1991). Transcriptional regulation by NF-κB can suppress TNF-α-induced apoptosis and may be the dominant pathway following TNF-α treatment (Beg and Baltimore, 1996; Hsu, Sho et al., 1996; Liu, Hsu et al., 1996; Van Antwerp et al.,1996; Wang, Mayo et al., 1996). In fact, many cells require the presence of RNA or protein synthesis inhibitors to elicit TNF-αmediated cell death (Rubin et al., 1988; White et al., 1992). Pretreatment of cells with interleukin-1, which induces NF-κB activation, protects from TNF-α-induced apoptosis (Wang, Mayo et al., 1996). Furthermore, overexpression of Rel A, one of the common components of NF-κB, protects cells from TNF-α (Beg and Baltimore, 1996). NF-κB is normally sequestered in the cytoplasm by binding to the inhibitory protein IκB (Baeurle and Baltimore, 1996). Phosphorylation of IκB leads to its degradation freeing NF-κB so that it can be translocated to the nucleus and regulate gene expression (Baeurle and Baltimore, 1996). Inhibition of NF-κB, either by disrupting one of its subunits (e.g. Rel A) (Beg and Baltimore, 1996) or by expression of an IκB mutant that is resistant to degradation (Van Antwerp et al., 1996; Wang, Mayo et al., 1996), facilitates TNF-α-induced apoptosis. The biochemical mechanism in which TNF-α activates NF-κB is somewhat unclear. NF-κB activation is not blocked by dominant negative mutants of FADD or by CrmA (Chinnaiyan, Tepper et al., 1996), suggesting that the pathway is different than that used for apoptosis. It is, however, triggered by overexpression of downstream proteins RIP or TRAF2 which are both recruited by TRADD to TNFR1 in a TNF-α dependent process (Hsu, Huang et al., 1996; Hsu, Shu et al., 1996). The binding of TRAF2 to TRADD, which occurs through its RING finger domain, is facilitated by interaction with the kinase domain of RIP (Hsu, Huang et al., 1996) Interestingly, it is the kinase domain of RIP that appears to be mediate activation of NF-κB (Ting et al., 1996). Dominant negative mutants of TRAF2 which lack the RING finger motif block TNF-α-induced NF-κB, but do not block apoptosis (Hsu, Shu et al., 1996). In fact, these mutants actually promote TNF-α-mediated cell death, further supporting the role of the NF-κB pathway in suppressing apoptosis (Hsu, Shu et al., 1996). This pathway was further characterized by the identification of a MAP (mitogen activated protein)-like kinase, NIK (NF-κB-inducing kinase), that binds to TRAF2 and promotes NFκB activation and inhibition of cell death (Malinin et al., 1997). However, the downstream effectors of NIK as well as the transcriptional targets for NF-κB remain unknown. Conclusion We have described several of the basic components of the apoptotic program including specific pathways initiated by TNF-α and Fas. In just the past year, we have begun to understand the structure of Bcl-2 family members and the possibility that they function as membrane spanning ion channels. A biochemical connection has been made between the Bcl-2 family and caspases. Although this junction is via the C. elegans protein Ced-4, it will probably not be too long before we realize its mammalian functional homologues. Finally, the recruitment of several proteins to the TNFR1/Fas complex has resulted in the identification of an effector protein, FLICE, which links this pathway to other apoptotic


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Part 1 INDUCERS OF APOPTOSIS

1. THE DEATH RECEPTORS KLAUS SCHULZE-OSTHOFF*,†AND MARCUS E.PETER** *Department of Internal Medicine I, Medical Clinics, University of Tübingen **Tumor Immunology Program, German Cancer Research Center, Heidelberg, Germany

INTRODUCTION Higher organisms have developed elaborate mechanisms to rapidly and selectively eliminate unwanted cells by programmed cell death. Exposure or depletion of certain steroid hormones, incubation with noxious agents, loss of cell adhesion to the extracellular matrix, or dysregulated expression of oncogenes are only some of the conditions that can lead to cell death. A fine-tuned mechanism to regulate life and death of a cell is the interaction of surface receptors with their cognate ligands, which may either trigger a survival or, oppositely, an apoptogenic signal. Several receptors are able to transmit cytotoxic signals into the cytoplasm, but in most cases they have a wide range of other functions unrelated to cell death. The T and B cell receptors, CD2, CD4 and cytokine receptors, such as those for interferons, TGF-β or TNFrelated ligands, are examples of molecules that induce diverse signals resulting in cell activation, differentiation, proliferation, or induction of apoptosis. Whether the signals induced by a given receptor lead to cell activation or death is highly celltype specific and tightly regulated during differentiation of cells. For example, TNF receptors can exert costimulatory signals for proliferation of naive lymphocytes as well as can induce death signals required for deletion of activated lymphocytes. Many receptors with important functions in differentiation, survival and cell death belong to an emerging family of structurally related molecules, called the TNF/NGF receptor superfamily (Figure 1.1). For some members of the family an apoptosis-inducing activity has been reported. However, most of them also have other functions such as induction of proliferation, differentiation, immune regulation and gene expression. Receptors with pleiotropic functions include TNF-R1 (CD120a) (Loetscher et al., 1990; Schall et al., 1990;

† Corresponding Author: Department of Internal Medicine I, Medical Clinics, Eberhard-KarlsUniversity, Otfried-Müller-Str.10 D-72076, Tübingen, Germany. Tel.: +49–7071–29 84113. Fax: +49–7071–29 5865.

THE DEATH RECEPTORS 33

Figure 1.1 Members of the tumor necrosis factor receptor superfamily. The subfamily of the death receptors is boxed. The tinted box in the cytoplasmic regions of the receptors represents the death domain (DD). Members of the family that have been shown to induce apoptosis without having a DD are grouped in the stippled box.

Smith et al., 1990), TNF-R2 (CD120b) (Dembic et al., 1990), CD40 (Stamenkovic et al., 1989), CD30 (Durkop et al., 1992), CD27 (Camerini et al., 1991), OX-40 (Mallett et al., 1990), 4–1BB (Kwon and Weissman, 1989), NGF-R (Radeke et al., 1987), TRAMP (DR3/ wsl-1/ APO-3/LARD) (Chinnaiyan et al., 1996a; Bodmer et al., 1997; Kitson et al., 1996; Marsters et al., 1996a; Screaton et al, 1997), HVEM (ATAR/TR2) (Montgomery et al., 1995; Hsu et al., 1997a; Kwon et al., 1997), GITR (Nocentini et al., 1997) and RANK (Anderson et al., 1997). The receptor molecules of this family are type I membrane proteins and are structurally homologous. Each contains in its extracellular domain two to six imperfect repeats of about 40 amino acids, each containing approximately six cysteine residues. Their cytoplasmic domains generally lack sequence homology. APO-1/Fas, now called CD95, was the first member of the TNF/NGF receptor superfamily described in terms of its function in apoptosis (Itoh et al., 1991; WatanabeFukunaga et al., 1992a; Oehm et al., 1992). Sequence comparison of the intracellular domain of CD95 with TNF-R1 revealed that both receptors contained a homologous stretch of about 80 amino acids. This region has been designated the ‘death domain’ (DD) since it enables transmission of a cytotoxic signal by both molecules (Tartaglia et al., 1993a; Itoh and Nagata, 1993). Recent homology searches in EST databases led to the cloning of a number of novel membrane receptors that contain such a death domain and are therefore referred to as the death receptors (DRs) (Figure 1.1). TRAMP (DR3/wsl-1/APO-3/LARD) is both structurally and functionally similar to TNF-R1 and is abundantly expressed in T lymphocytes (Chinnaiyan et al., 1996a; Bodmer et al., 1997; Kitson et al., 1996; Marsters et al., 1996a; Screaton et al., 1997).TRAIL-R1 (DR4, APO-2) and TRAIL-R2 (DR5) have been found as receptors binding to a novel cytokine, called TRAIL (TNF-related apoptosisinducing ligand). The two TRAIL receptors are functionally similar to CD95 as their main function seems to be to induce apoptosis (Pan et al., 1997a; 1997b; Sheridan et al., 1997; Walczak et al., 1997; MacFarlane et al., 1997). They are the only TNF-R-like molecules possessing only two cysteine-rich domains. The TRAIL system, in addition, consists of two neutralizing decoy receptors, called DcR1 (TRAIL-R3, TRID) and DcR2 (Pan et al., 1997b; Sheridan et al., 1997; Degli-Eposti et al., 1997; MacFarlane et al., 1997; Marsters et al.,

34 K.SCHULZE-OSTHOFF AND M.E.PETER

1997). The sequence of DcR1 encodes a protein that contains the external TRAIL-binding region as well as a glycosyl-phosphatidylinositol residue that anchors the receptor to the membrane. But unlike the other receptors, DcR1 lacks an intracellular tail needed to spark the death pathway. DcR2 is also able to bind TRAIL but contains a truncated death domain. Thus, both decoy receptors will prevent TRAIL from engaging functional TRAIL receptors and thereby may render cells resistant to apoptosis. Collectively, this underlines that the death domain is required to induce apoptosis triggered by the different surface receptors. In this review, we will survey the function of the death receptors and their respective ligands with special emphasis on the signal transduction pathways used by these receptors. THE DEATH LIGANDS For most members of the TNF-R superfamily their cognate ligands have been identified. Five of them, CD95L, TNF, lymphotoxin-α (LTα, TNFβ), TRAIL and APO-3L (TWEAK) bind to death receptors (Suda et al., 1993; Beutler and van Huffel, 1994; Wiley et al., 1995 Marsters et al., 1998). It was not surprising to find that, in addition to the receptors, also the ligands display striking structural homologies, which are reflected by similar mechanisms of receptor recognition and triggering. With the exception of NGF, all ligands recognize their receptors through a shared structure composed of anti-parallel β-sheets arranged in a jelly roll structure. The crystal structure of TNF and LTα alone or LTα in complex with the extracellular domain of the TNF-R1 revealed a trimeric structure (Eck and Sprang, 1989; Eck et al., 1992; Jones et al., 1992; Banner et al., 1993). Together with a number of biochemical data (Karpusas et al., 1995; Pitti et al., 1996), it is believed that all active ligands have a trimeric structure in solution and activate their cognate receptors by trimerization. Data on the CD95 receptor demonstrating that dimerization of CD95 was not sufficient to trigger apoptosis supported this notion (Dhein et al., 1992; Kischkel et al., 1995). Another common feature of the ligands is that almost all of them are synthesized as type II transmembrane proteins. The only exception is LTα which, although formed as a soluble protein, can bind as a subunit to another membrane-bound ligand of the family, LTβ, and also act as a cellbound form. Lymphotoxins can be found as homo-(LTα3) or heterotrimers (LTα1/β2 or LTα2/β1). The LTα homotrimer binds the TNF receptors, whereas the heterotrimers bind to the LTβ receptor which does not contain a death domain. Although TNF-related ligands are synthesized as membrane-bound molecules, for most of them soluble forms have been identified. The secreted forms of the ligands are generated by the activity of rather specific metalloproteinases. This was suggested for CD95L (Kayagaki et al., 1995; Mariani et al., 1995). For TNF, a zinc-dependent metalloprotease, called TACE (TNFα-converting enzyme) was recently cloned and shown to specifically cleave TNF (Black et al., 1997; Moss et al., 1997). Activation of a death receptor pathway is in many cases controlled by the inducible de novo expression of the respective death ligands such as TNF, CD95L or TRAIL. As described below, several apparently different deathinducing conditions, such as the exposure to chemotherapeutic agents or irradiation, can induce the expression of death ligands and may therefore, at least partially, mediate cell death via receptor-ligand interaction.


BIOLOGICAL FUNCTIONS OF DEATH RECEPTORS TNF and LTα were isolated more than 15 years ago, on the basis of their ability to kill tumor cells in vitro and to cause hemorrhagic necrosis of transplantable tumors in mice. Because TNF proved to be highly toxic in animals and humans, it did not fulfill initial expectations that would be useful in the treatment of cancer. Considerable evidence suggests that overproduction or inappropriate expression of TNF plays a role in various chronic inflammatory disorders, modifies anticoagulant properties of endothelial cells, activates neutrophils and induces cytokine expression. Bone resorption, fever, anemia and wasting may all, in some measure, be attributable to TNF (reviewed in Tracey and Cerami, 1993; Bazzoni and Beutler, 1996; Fiers et al., 1996). The CD95 (APO-1, Fas) molecule has been identified much later as a cell surface receptor that could mediate apoptotic cell death of transformed cells and cause regression of experimental tumors growing in nude mice (Trauth et al., 1989; Yonehara et al., 1989). Although due to the high systemic side effects, application of CD95-mediated apoptosis has also not hold its promise as a potential cancer therapy, several evidences have now documented a pivotal role of CD95 in various physiological and pathological forms of cell death (reviewed in Schulze-Osthoff, 1994; Krammer et al., 1994; Nagata and Golstein 1995; Nagata, 1997). Finally, TRAIL and its different receptors have been identified as the last, but probably not least, receptor-mediated apoptosis system. TRAIL is able to induce apoptosis in many transformed cells. In contrast to TNF and CD95L, no side effects of TRAIL on normal primary cells have yet been reported. Thus, whether the TRAIL pathway represents the long-sought-after means to selectively kill tumor cells remains to be shown. In the following, we will describe the relevance and some biological functions of these receptors and their ligands within the organism. Although most functions have been described in the immune system, death receptors are involved in a number of very different apoptotic settings ranging from cell homeostasis, organ development, immune privilege and anticancer treatment. The apoptotic machinery utilizing death receptor/ligand systems is very powerful and requires tight regulation. Disturbance of these systems can cause severe disease. The CD95 system The importance of the CD95 system has mainly been addressed in the immune system. Our understanding of the function of the CD95 receptor and its ligand has greatly been enhanced by the finding that both molecules are mutated in mouse strains suffering from severe autoimmune diseases. Lpr (for lymphoproliferation) mice which lack a functional CD95 receptor (Watanabe et al., 1992b) as well as gld (for generalized lymphoproliferative disease) mice which bear a mutant CD95 ligand (Takahashi et al., 1994a) exhibit various autoimmune phenomena resembling systemic lupus erythematosus in men. Both mouse strains produce autoantibodies and accumulate CD4-CD8-T cells leading to lymphadenopathy, splenomegaly and other signs of autoimmune disorders. The lpr mutation is caused by the insertion of a transposable element into intron 2 of the gene encoding CD95, thereby preventing full-length transcription (Adachi et al., 1993). The gld defect


arises from a point mutation within the CD95L gene, changing an amino acid critical for CD95 binding. Recently, in men a similar disease with a dysfunction of the CD95 system was reported (Fisher et al., 1995; Rieux-Laucat et al., 1995; Drappa et al., 1996). Children with “autoimmune lymphoproliferative syndrome” (ALPS), also called Canale-Smith syndrome, have massive nonmalignant lymphadenopathy, hepatosplenomegaly, altered and enlarged T cell populations and other manifestations of systemic autoimmunity. The loss-of-function phenotype in mice and men indicates that CD95 plays an important role in the regulation of a normal immune response and the maintenance of self-tolerance. Molecular studies provided evidence that CD95 is the mediator of activationinduced cell death (AICD), a form of apoptosis important for the downsizing of the immune response, as well as an effector component of cytotoxic T cell activity (Dhein et al., 1995; Brunner et al., 1995; Ju et al., 1995; Rouvier et al., 1993; Hanabuchi et al., 1994; Alderson et al., 1995). In contrast, there is no convincing evidence so far to believe that CD95 is involved in negative selection. This assumption is consistent with a relatively normal thymic architecture and proper thymic deletion of superantigen-activated T cells in lpr mice (Herron et al., 1993; Singer and Abbas, 1994). lpr and gld mice, in addition to abnormalities in the T cell compartment, show B cell hyperreactivity associated with the production of autoantibodies, suggesting that CD95 also controls the expansion of the B cell compartment. Indeed, it has been found that CD40 ligand sensitizes B cells to CD95-mediated apoptosis, whereas CD40 ligation plus engagement of surface immunoglobulin protects cells. This indicates that the CD95L-mediated demise may represent a mechanism to prevent nonspecific B cell activation and confer antigen specificity to the interaction of helper T cells with B cells (reviewed in Krammer et al., 1994; Cornall et al., 1995). Just as a defect of the CD95 system is intimately linked to autoimmune diseases caused by the impaired removal of autoreactive lymphocytes, so may inappropriate induction of apoptosis lead to various pathological conditions. Accumulating evidence exists that CD95 is critically involved in the progression of viral diseases, such as HIV-1 or hepatitis virus infections where massive apoptosis occurs. It could be shown that indirect mechanisms lead to a sensitization of noninfected T cells towards AICD after HIV-1 infection (Westendorp et al., 1995; Li et al., 1995a; Szawlowski et al., 1993; Zagury et al., 1993). T lymphocytes from HIV-1 infected patients exhibit an elevated expression of CD95 and sensitivity towards CD95-mediated apoptosis (Debatin et al., 1994; Katsikis et al., 1995). Two HIV-1 derived soluble proteins, gp120 and Tat, have been found to activate the inducible expression of CD95L in T lymphocytes (Westendorp et al., 1995). This event may then cause a fratricide or suicide death of uninfected T lymphocytes and result in the continuous depletion of CD4 + T cells during AIDS disease. Although CD95L has been originally found on activated T lymphocytes, various other non-lymphoid cells can express CD95L. A high constitutive express ion can be detected in Sertoli cells of the testis and epithelial cells of the anterior eye chamber (Griffith et al., 1995; Bellgrau et al., 1995). This finding led to the proposal that CD95L may be responsible for the maintenance of immune privilege, which characterizes the ability of certain organs to suppress graft rejection, even when transplanted in non-matched individuals. After viral inoculation into the anterior eye chamber, infiltrating lymphocytes and granulocytes undergo apoptosis probably due to high expression of CD95L on epithelial cells (Griffith et al., 1995; 1996). This apoptosis is not observed in eyes of animals with defective CD95L


(gld mice), and the resulting uncontrolled inflammation destroys the tissue. Thus, CD95L is necessary for the maintenance of the privileged status of the eye by killing infiltrating lymphocytes of the host. It is interesting to note that gld and lpr mice have no apparent ocular abnormalities and no increased lymphocytic infiltration. Presumably, even with aberrant CD95L expression, organs such as the eye maintain their function through other additional mechanisms that insure immune privilege. The exciting novel function of CD95 in immune privilege has presumably enormous practical implications for future transplantation strategies aimed to avoid allograft rejection. It was recently shown that human corneas express functional CD95L (Stuart et al., 1997), raising the possibility that this molecule could act to protect cornea grafts. Examination of corneal transplants in mice supported this idea; while approximately 45% of allogeneic cornea transplants survived for an extended period, no graft survival was seen with corneas expressing defective CD95 receptor or ligand. Thus, the protection of allogeneic grafts was dependent upon the presence of a functional CD95 system. Other studies on the CD95L involvement in graft acceptance are less clear and currently very controversially discussed. A protective effect of CD95L expressed in the testis was observed after transplantation of allogeneic testis under the kidney capsule (Bellgrau et al., 1995), but this result could not be confirmed by others (Allison et al., 1997). In another example, syngeneic myoblasts expressing ectopic CD95L protected allogeneic pancreatic islets co-implanted under the kidney capsule of animals with streptozotocin-induced diabetes (Lau et al., 1996). These grafts, which were quickly rejected if myoblasts did not express CD95L, maintained their function for an extended period of time. Consistent with this was the observation that allogeneic islets showed delayed rejection when co-implanted with CD95L-expressing testis tissue (Selawry and Cameron, 1993). However, in other studies no protective effect of CD95L was observed. Allison et al., (1997) reported that transgenic expression of CD95L in pancreatic islets failed to protect these from allogeneic transplant rejection when placed under the kidney capsule. The presence of CD95L rather induced a granulocytic infiltrate in the animals, which damaged but did not destroy the islets. The finding is related to a report showing that CD95L on tumor cells can induce a granulocyte-mediated rejection reaction(Seino et al., 1997). CD95L-mediated depletion of cytotoxic T lymphocytes may not only be beneficial but may also play a role for tumor cells to escape the host’s immunosurveillance. Recently, high constitutive CD95L expression has been found in distinct lineages of tumors, such as colon, lung, renal carcinoma, melanoma, hepatocellular carcinoma, astrocytoma and T- and B-cell derived neoplasms (O’Connell et al., 1996; Hahne et al., 1996; Strand et al., 1996; Niehans et al., 1997; Saas et al., 1997; Shiraki et al., 1997; Tanaka et al., 1996; Vfflunga et al., 1997; unpublished results). This suggested that the same mechanisms responsible for protecting tissues from autoimmune destruction during inflammation and graft rejection may be also used by tumors in establishing immunologically privileged environments. An increasing body of data implies a role of CD95 in inflammatory situations and autoimmune diseases that are associated with tissue destruction. Most likely CD95L expression causes tissue damage directly, or indirectly by recruitment of granulocytes. In graft-versus-host disease, the ability of the graft effector cells to express functional CD95L contributes to the destructive assault (Baker et al., 1996; Braun et al., 1996). The observation that anti-CD95 antibody induces apoptosis in hepatocytes in vivo led to the idea


that CD95L-induced apoptosis may be involved in some forms of hepatitis (Ogasawara et al., 1993). This was supported at least in patients with alcoholic liver damage, in which the hepatocytes express CD95L (Galle et al., 1995). There is also some evidence for a role of CD95L-induced apoptosis in autoimmune diseases, such as diabetes and encephalomyelitis (Chervonsky et al., 1997; Sabelko et al., 1997; Waldner et al., 1997). A particular situation was found in the thyroid gland. Normal thyrocytes constitutively express functional CD95 ligand, but do not express the receptor. However, in Hashimoto’s thyroiditis patients, thyrocytes do express CD95, and these cells undergo apoptosis (Giordano et al., 1997). In vitro, normal thyrocytes express CD95 after exposure to IL-1, and the resulting apoptosis can be blocked by antibodies that disrupt CD95/CD95L interactions. Hence, in Hashimoto’s thyroiditis the normally protective function of CD95L on thyrocytes leads to the destruction of the thyroid gland. The cause for this dysfunction is unclear, but it is likely that the CD95 system contributes to the disease process. Although CD95 has a predominant function in inducing apoptosis, there are some conditions where CD95L may obviously trigger an inflammatory reaction. In this respect, it has been found that CD95 ligation can result in secretion of the chemokine IL-8 (AbreuMartin et al., 1995). In conjunction with T-cell receptor activation or other signals in T cells, CD95 may act as a costimulatory molecule, enhancing gene expression of IL-2 and other cytokines (Alderson et al., 1993). In addition, some cell types can respond to CD95 ligation via proliferation and not celldeath(Alderson et al., 1993; Mapara et al., 1993; Aggarwal et al., 1995; Freiberg et al., 1997). However, the overall in vitro and in vivo data suggest that CD95 is a receptor which mainly mediates apoptosis. This is also reflected by the observation that, in contrast to TNF, CD95 does generally not induce the activation of proinflammatory transcription factors, including NF-κB, AP-1 or NF-AT (SchulzeOsthoff et al., 1994). An exciting finding was that several unrelated death-inducing agents and conditions obviously utilize physiological means of induction of apoptosis. Apoptosis mediated by p53 may involve the CD95 system, as the gene encoding CD95 has been found as a putative target of this transcription factor (Owen-Schaub et al., 1995). Also overexpression of the cMyc proto-oncogene, which induces cell death under growth-limiting conditions, appears to mediate death, at least partially, by a mechanism requiring CD95/CD95L interaction (Huber et al., 1997). An apoptosisinducing effect of c-Myc was not observed in lpr and gld mice. Furthermore, AICD in mature and immature T lymphocytes is blocked by c-myc antisense oligonucleotides (Bissonnette et al., 1994). The mechanism how c-Myc sensitizes cells for CD95-mediated apoptosis remains to be elucidated. Recent data demonstrate that anticancer drug-induced cell death may involve the CD95 system. Several different drugs, widely used in chemotherapy of cancers induce CD95L expression in leukemic, hepatocellular and neuroblastoma cells (Friesen et al., 1996; Müller et al., 1997; Fulda et al., 1997a). Binding of CD95L to the receptor then triggers the apoptosis cascade in chemosensitive tumor cells. In support of these data, anticancer druginduced cell death has been found in most cases to be inhibitable by CD95L neutralizing reagents. The upregulation of CD95L may therefore provide an new clue to the mechanism of action of chemotherapeutic drugs. Still another situation has been found in cell death induced by UV irradiation. UVinduced apoptosis is strongly attenuated in CD95-resistant cells (Rehemtulla et al., 1997;


Aragane et al., 1998). It has been shown that UV irradiation directly oligomerizes and thereby activates death receptors, such as CD95 and TNF-R1. This is presumably mediated by energy transfer which may allow for a subsequent conformational change of the receptors (Rosette and Karin, 1996; Aragane et al., 1998). UV-induced apoptosis therefore does not require CD95L de novo expression, but directly engages the CD95 signalling pathway. Altogether, these findings demonstrate that CD95 and presumably other death receptors play a role in very diverse apoptosis settings. The TNF system In contrast to the CD95 system, the biological function of the TNF/TNF-R system is much more complex. In addition of being cytotoxic for mainly transformed cells, TNF exerts a number of other activities related to proinflammatory processes on almost all cell types (reviewed in Fiers et al., 1996; Bazzoni and Beutler, 1996). The function of TNF is also complicated by the fact that two different TNF receptors, TNF-R1 and TNF-R2, exist which can be occupied by two different ligands. TNF and the LTα homotrimer bind to the TNF receptors, while LTα/LTβ complexes selectively ligate the LTβ receptor. TNF was originally found as a serum factor in endotoxin-primed mice which caused hemorrhagic necrosis of transplanted tumors. Almost concurrently, the factor was identified as a catabolic substance that suppressed the expression of lipoprotein lipase and other anabolizing enzymes in fat and was therefore termed cachectin. The gene encoding TNF was cloned by several groups and found to cluster with the genes for LTα and LTβ in the major histocompatibility complex. A major cellular source of TNF are activated macrophages, but also other cell types such as lymphoid cells, NK cells, neutrophils, keratinocytes, fibroblasts and smooth muscle cells produce the cytokine in response to inflammatory and environmental challenges. The inducible expression of TNF is regulated at the transcriptional but also translational level. Whereas TNF can be secreted by a variety of cell types, the lymphotoxins are mostly produced by activated lymphocytes and NK cells. Though the principal interest in TNF arose from its antitumor activity, it soon became clear that TNF has a wide range of other biological effects and is a mediator of endotoxic shock. An important cellular target of TNF action is the endothelium where TNF induces the release of platelet activating factor (PAF), the secretion of various cytokines, such as IL-1, IL-6, IL-8, GM-CSF, and the expression of adhesion molecules including ICAM-1, VCAM-1 and E-selectin. These responses together with the activation of arachidonic acid metabolism commonly result in increased vascular permeability, anticoagulant activity and leukocyte adhesion. Because TNF receptors are ubiquitously expressed with the exception of erythrocytes, it is not suprising that almost all cell types respond to TNF. In neutrophils, TNF activates respiratory burst and degranulation leading to the release of reactive oxygen intermediates, elastase, lysozymes and other granular enzymes. Macrophages respond to TNF with enhanced cytotoxic activity and cytokine synthesis. In hepatocytes, TNF is, together with IL-1 and IL-6, an important mediator of the synthesis of acute-phase proteins. In addition, a multitude of biological effects have been described for several cell types including osteoblasts, fibroblasts, smooth muscle cells and others. Many of the proinflammatory activities of TNF are regulated by the transcription factor NF-κB.


Most of the biological activities of TNF including programmed cell death, antiviral activity, and activation of transcription factor NF-κB, are mediated by TNF-R1, while an involvement of TNF-R2 has been demonstrated particularly in T lymphocytes (Engelmann et al., 1990; Espevik et al., 1990; Tartaglia et al., 1991; 1993b; Wong et al., 1992; Vandenabeele et al., 1995). Membrane-bound TNF (mTNF) and soluble TNF (sTNF) have different affinities to the two receptors, with TNF-R2 preferentially binding mTNF (Grell et al., 1995). TNF-R2 appears to play an auxiliary role in cellular responses to sTNF. It has been suggested that TNF-R2-bound ligand may be passed over to TNF-R1 to enhance TNFR1 signalling. This process, termed ligand passing, is favored by the distinct kinetics of ligand association and dissociation at the two receptors. TNF binding to TNF-R2 has a fast off-rate that creates a locally high TNF concentration at the cell surface, which in turn facilitates binding to TNF-R1 which has a slow dissociation rate (Tartaglia et al., 1993b). The prime physiological activator of TNF-R2 seems to be mTNF, since TNF-R2 can be more strongly stimulated by mTNF rather than by sTNF. As mTNF also signals via TNF-R1, the resulting cooperativity of both receptors leads to cellular responses much stronger than those achievable with sTNF alone. Moreover, it was shown that upon appropriate activation of TNF-R2, a switch of the cellular response pattern to TNF occurred, such that, as an example, cells fully resistant to the cytotoxic action of sTNF become highly susceptible upon contact with mTNF (Grell et al., 1995). Gene targeting and transgene technologies have been used to unravel the in vivo role of the TNF system and to establish genetically defined models of human diseases. TNF(−/−) mice show an almost normal phenotype histologically, but have reduced sensitivity to LPSmediated toxicity and increased sensitivity to intracellular pathogens such as Listeria and Candida, due to severely impaired macrophage functions (Pasparakis et al., 1996; Marino et al., 1997). Apart from their deficiency in effector functions, TNF(−/−) mice have defects in lymphoid organogenesis and formation of germinal centers. Gene targeting of LTα also results in a lack of primary and secondary lymphoid follicles, but moreover these mice have a defective lymphnode development (reviewed in von Boehmer et al., 1997). The importance of mTNF in vivo has been elegantly demonstrated in several transgenic models. For example, it was reported that the deficiencies of TNF(−/−) mice are reconstituted by ectopical expression of noncleavable mTNF (Korner and Sedgwick, 1996). In a different transgenic model it was shown that mTNF induces multiple sclerosis-like disease with paralysis and a histopathology resembling experimental autoimmune encephalomyelitis when expressed in microglia, but not neuronal cells. In contrast to TNF-R1(–/–) mice (Rothe et al., 1993), deletion of TNF-R2 has no apparent influence on lymphoid organ development. TNF-R2 is critically involved in mediating pathogenicity during cerebral malaria, is essential for LPSinduced leukostasis and downregulates TNF-R1 dependent neutrophil influx in a lung inflammation model (Garcia et al., 1995). A dominant role of TNF-R1 in mediating pathogenic activities was evident early on from models of septic shock and arthritis (Espevik and Waage, 1988; Shimamoto et al., 1988; Hayward and Fiedler-Nagy, 1987). The growing knowledge about the pathophysiological role of TNF in acute and especially in chronic diseases calls for strategies to intervene with the deleterious effects of TNF. Clinical trials employing anti-TNF reagents have been impressingly successful in diseases such as rheumatoid arthritis, septic shock and inflammatory bowel diseases (Maini, 1996; Glauser, 1996; Stokkers et al., 1995).


The TRAMP and TRAIL Systems The biological functions of the new death receptors are largely undefined at present. TRAMP (TNF receptor-related apoptosis-mediating protein, DR3) is abundantly expressed in thymocytes and lymphocytes and may therefore play a role in lymphocyte development (Chinnaiyan et al., 1996a; Bodmer et al., 1997). The chromosomal localization of the TRAMP gene has been assigned to the long arm of chromosome 11 where other related receptors (CD30R, TNFR2, OX40R) have been mapped. TRAMP is both structurally and functionally related to the TNF receptors, because its overexpression or stimulation by its recently identified ligand APO-3L (TWEAK) (Chicheportiche et al., 1997; Marsters et al., 1998) leads to NF-κB activation and apoptosis. The recent cloning of TRAIL and its three receptors revealed a new apoptosis system with apparently high complexity. Among the ligands of the TNF family, TRAIL is most closely related to CD95L. However, in contrast to the restricted expression of CD95L, TRAIL is more abundantly expressed in several tissues. Two of the TRAIL receptors, TRAIL-R1 and TRAIL-R2, can induce apoptosis in various cancer cells, whereas the decoy receptors DcR1 and DcR2, which are mostly expressed on normal cells, do not contain a functional death domain and therefore confer resistance against TRAIL action (Pan et al., 1997a; 1997b; Sheridan et al., 1997; Walczak et al., 1997; Degli-Eposti et al., 1997; MacFarlane et al., 1997; Masters et al., 1997). Whether TRAIL might be used to kill more selectively tumor cells, awaits further in vivo experiments. Because TRAIL only weakly triggers NF-κB activation, it can be expected that in vivo administration of TRAIL will not lead to severe side effects related to proinflammatory gene expression. At the moment, it is unclear why there are two death-signalling TRAIL receptors. It is possible that either the two TRAIL receptors are redundant or provide an additional and versatile means to regulate apoptosis. So far, there is only limited information on target cells of TRAIL-induced apoptosis. It was reported that TRAIL can induce AICD in activated T cells (Marsters et al., 1996a; 1996b). Furthermore, it has been shown that TRAIL-induced AICD may contribute to cell death of T lymphocytes during HIV infection (Katsikis et al., 1997). DEATH RECEPTOR-ASSOCIATING MOLECULES A major advancement in our understanding of death receptor signalling was the definition of the so-called death domain (DD) within the death receptors, which is required for the transmission of a cytotoxic signal. The DD has been characterized in detail in TNF-R1 and CD95 and consists of a stretch of about 80 amino acids that are essential for triggering cell death. Its importance is also demonstrated by lprcg mice which carry a point mutation in the DD of CD95. Delineation of the DD was not only a major aid for the identification of new death receptors, when used as a screen in EST databases, but also allowed for the identification of new adaptor molecules, when used as a bait in interactive cloning approaches. The DD exerts its effects via interactive properties. As part of some proteins, it was found to self-associate and to be capable of binding to the DD in other proteins. These associations between DDs occur as a consequence of receptorligand binding and seem to involve electrostatic interactions. NMR spectroscopy of the DD of CD95 confirms that this


region, which comprises a series of antiparallel amphipathic α-helices, has many exposed charged residues (Huang et al., 1996). The tendency of the DD to self-associate apparently fortifies the interactions of the receptors imposed by ligand binding. Following selfassociation, the DD of the receptors recruits and binds other DD-containing proteins which then serve as adaptors in the signalling cascades (Figure 1.2). The first DD-containing adaptor proteins identified were FADD (MORT1) (Chinnaiyan et al., 1995; Boldin et al., 1995), RIP (Stanger et al., 1995) and TRADD (Hsu et al., 1995). TRADD is most effectively bound following ligation of TNF-R1, where it then likely serves to recruit the DD proteins FADD and RIP as well as the RING domain adaptor protein TRAF2. FADD, in contrast, is preferentially recruited to CD95. Thus, the DD of FADD can bind to the DD of TRADD and the DD of RIP to the DDs of both TRADD and FADD. These mutual interactions may therefore account for a potential crosstalk of the different death receptor signalling pathways. Overexpression of DD proteins causes cell death indicating that these molecules are involved in apoptosis signalling. In the case of FADD, transient expression of the N-terminal region was found to be sufficient to cause apoptosis (Chinnaiyan et al., 1995). This part of FADD was therefore termed the death effector domain (DED) (Chinnaiyan et al., 1996b). In contrast, overexpression of the C-terminal DD-containing part lacking the DED (FADDDN) protected cells from CD95-mediated apoptosis and functioned as a dominant-negative mutant. This suggested that the N-terminus of FADD is coupled to the cytotoxic machinery. Both TRADD and RIP induce apoptosis but can also activate NF-κB, which is a typical feature of TNF-induced signalling (Hsu et al., 1995; 1996a; Park et al., 1996; Ting et al., 1996). Similar to FADD, the C-terminus of TRADD contains a DD enabling selfassociation and association with the DD of other signalling molecules including TNF-R1 and FADD. TRADD, however, lacks the typical DED present in FADD. RIP (receptor-interacting protein) contains an N-terminal kinase domain and a C-terminal DD. It was originally identified as a molecule binding to the cytoplasmic domain of CD95 in a two-hybrid system (Stanger et al., 1995). Therefore, RIP was suggested to play a role in CD95-signalling. Later however, it was demonstrated that RIP does not directly bind to CD95 or TNF-R1 but is recruited through the TNF-R adaptor protein TRAF2. RIP was identified to be crucial for TNF-R1-mediated NF-κB activation. In a mutant cell line that had lost expression of RIP, CD95 signalling was not affected, whereas TNF-R1-mediated NF-κB activation was blocked (Ting et al., 1996). After reconstitution with RIP, NF-κB activation in this cell line was restored. The intermediate region of RIP was found to be responsible for activating NF-κB which was potentiated by the presence of the kinase region and DD. While most of the information regarding death pathways has been obtained from yeast twohybrid assays or supra-physiological overexpression of DD proteins in mammalian cells, for CD95 the signalling complexes have been also identified in vivo using classical biochemical methods (Kischkel et al., 1995). Treatment of cells with the agonistic mAb anti-APO-1 and subsequent co-immunoprecipitation of CD95 resulted in the identification of four cytotoxicity-dependent APO-1-associated proteins (CAP1-4) on two-dimensional gels within seconds after receptor triggering. Together with the receptor these proteins formed the death-inducing signalling complex (DISC). Using specific antisera two of the proteins (CAP1 and 2) were identified as two different serine phosphorylated species of FADD and it was demonstrated that FADD bound to CD95 in a stimulation-dependent fashion in vivo.


Figure 1.2 Proposed signalling pathways of CD95-mediated apoptosis. One important pathway includes recruitment of the adapter protein FADD through interaction between the death domains (DD) of FADD and CD95. The death effector domain (DED) of FADD in turn recruits procaspase-8 which is cleaved and activated at the DISC. An alternative pathway may involve activation of caspase-2 through the RIP-interacting protein RAIDD. RAIDD contains DD and a caspase recruitment domain (CARD) that is also present in procaspase-2. A third pathway may include recruitment of Daxx to the cytoplasmic domain of CD95. This pathway involves JNK activation and further downstream activation of caspases. It should be mentioned that the physiological relevance of the latter two pathways is rather unknown.

Sequencing of the other immunoprecipitated proteins, CAP3 and CAP4, resulted in the identification of a further downstream DED-containing protein. Using nanoelectrospray tandem mass spectrometry, sequence information of CAP3 and CAP4 was obtained that led to the retrieval of a full-length clone from a cDNA data base (Muzio et al., 1996). This protein contained two DEDs at its N-terminus, which associate with the DED of FADD. At its Cterminus it had the typical domain structure of an ICE-like protease and was therefore termed FLICE (FADD-like ICE). FLICE was also cloned by two other groups and named MACH and Mch5 (Boldin et al., 1996; Fernandes-Alnemri et al., 1996a). It belongs to cysteine proteases of the caspase family and is therefore now referred to as caspase-8 (Alnemri et al., 1996). Identification of caspase-8 as part of the DISC connected two different levels in apoptosis pathways, the receptor level with the level of the apoptosis executioners, the caspases. Another possible route for death receptor signalling was recently suggested by the discovery of a new death adaptor protein, called RAIDD or CRADD (Figure 1.2) (Duan and Dixit 1997; Ahmad et al., 1997). This protein contains a carboxy-terminal DD that binds to the DD of RIP. At its amino terminus RAIDD has a domain that is responsible for the


recruitment and binding to the prodomain of caspases, in particular caspase-2. The contribution of this pathway to TNF and CD95 cytotoxicity is still unknown. Furthermore, it was demonstrated that death receptors can directly trigger signalling pathways other than caspases. MADD was cloned as DD-containing protein that binds to TNF-R1 and activates ERK2, a MAP kinase member (Schievella et al., 1997). Similarly for CD95, a protein, called Daxx, was identified that specifically associates with the DD of CD95 (Yang et al., 1997). Overexpression of Daxx stimulates stress-activated protein kinases of the MAP kinase family (JNK/SAPK) and enhances apoptosis (Figure 1.2). Thus, a single receptor is able to trigger multiple pathways which, in addition to the FADD/caspase cascade, participate in induction of cell death. Despite the key role of the DD, the possibility that proteins associated with other intracellular regions of the receptors contribute to the overall pattern of apoptosis cannot be excluded. As will be described below, there are some evidences for such proteins, which either utilize other signalling mechanisms or which may suppress a death signal elicited at the DD. THE DEATH-INDUCING SIGNALLING COMPLEX (DISC) As described previously, caspase-8 was identified in vivo as a part of the CD95 DISC, suggesting that caspase-8 activation occurred at the DISC level. Indeed, it was recently confirmed that the entire cellular amount of cytoplasmic caspase-8 can be converted into active caspase-8 subunits at the DISC (Medema et al., 1997a). After receptor engagement, FADD and caspase-8 are recruited to CD95 within seconds. Binding of caspase-8 to receptorassociated FADD then presumably causes a structural change, resulting in autoproteolytic activation of the protease. The active subunits p10 and p18 are released into the cytoplasm whereas part of the prodomain remains bound to the DISC. Presently, it is assumed that active caspase-8 subunits cleave various death substrates including other caspases, such as caspase-3, which then leads to the execution of apoptosis. Using anti-caspase-8 antibodies it became clear that from all of the eight published caspase-8 isoforms two were predominantly expressed at the protein level (Scaffidi et al., 1997). These isoforms, termed caspase-8/a and caspase-8/b, are recruited and processed with similar kinetics at the DISC. Recently, it was demonstrated that recombinant caspase-8 lacking the prodomain could cleave caspase-8 in vitro suggesting an amplification step with caspase-8 at the top of a caspase cascade (Srinivasula et al., 1996a; Muzio et al., 1997). However, using the in vivo DISC assay, this could not be confirmed in intact cells (Medema et al., 1997a). It is possible that recombinant caspase-8 lacking the prodomain displays different substrate specificity in comparison to full-length caspase-8 in vivo. Overexpression of functionally inactive FADD and caspase-8 did not only block CD95, but also inhibited TNF-R1-induced signalling (Chinnaiyan et al., 1996b; Boldin et al., 1996). This suggested that both receptors are coupled to a similar signalling complex following cell activation. However, FADD does not directly bind to TNF-R1 but becomes associated upon binding of the DD-containing protein TRADD. Indeed, in cells stimulated with TNFα caspase-8 is processed, suggesting that TNF-R1 uses a similar signalling pathway for induction of apoptosis (unpublished data). So far a direct biochemical association has not been shown at the DISC level in intact cells. It remains therefore unclear whether caspase-8


is activated by association with TNF-R1 or whether its activation is the result of a secondary event. In contrast to CD95, TNF-R1 signalling seems to be more complex (see below). It was demonstrated that RAIDD/CRADD, another DD-containing protein, can bind to TNF-R1 in vitro or when overexpressed in 293 cells (Duan and Dixit, 1997; Ahmad et al., 1997). RAIDD/CRADD carries the DD at its C-terminus and at its N-terminus it displays homologies with the prodomain of caspase-2. It binds to TNF-R1 more efficiently in the presence of RIP. It was therefore suggested that RAIDD would engage TNF-R1 via RIP and induce activation of caspase-2. However, no in vivo data are available and the mechanism of caspase-2 activation remains to be determined. TRAMP has been reported to bind FADD, TRADD, TRAF2, and caspase-8 (Chinnaiyan et al., 1996a). Due to its TNF-R1-like structure it is expected to have a signalling function analogous to TNF-R1. Similarly to TNF-R1, TRAMP can induce apoptosis and activate NFκB (Chinnaiyan et al., 1996a; Bodmer et al., 1997; Kitson et al., 1996; Marsters et al., 1996b; Screzton et al., 1997). TRAIL is known to bind to two receptors, TRAIL-R1 and TRAIL-R2, both of which can signal for cell death. As with the other death receptors, TRAIL-R1 and TRAIL-R2-mediated apoptosis involves caspases, because caspase inhibitors as well as overexpression of CrmA or p35 blocked TRAIL-induced cell death (Pan et al, 1997a; 1997b; Sheridan et al., 1997; MacFarlane et al., 1997; Mariani et al., 1997). In fact, also caspase-8 activation was detected upon treatment sensitive BJAB cells with TRAIL (unpublished results). The data support the view that caspase-8 among other caspases is involved in the TRAIL-R signalling pathway. Moreover, it was recently suggested that caspase-10 (FLICE-2), which has a structure similar to caspase-8, is preferentially activated by TRAIL, whereas CD95 more strongly activates caspase-8 (Pan et al., 1997b). A number of reports indicate that the proximal signalling pathways of the TRAIL receptors are similar but distinct from CD95. For instance, Ag8 mouse myeloma cells have been found to be sensitive to TRAIL-induced, but not CD95Lmediated apoptosis, although CD95 was detected on the surface of these cells (Mariani et al., 1997). In addition, it was demonstrated that HeLa cells transfected with dominant-negative FADD became resistant to CD95 but remained sensitive to TRAIL (Marsters et al., 1996a). This finding indicates that FADD does obviously not play an obligatory role in TRAIL-induced apoptosis. It is now generally established that TRAIL-R1 does not couple to FADD. However, whether TRAILR2mediated cell death requires association with FADD remains controversial. Most reports point out that, similarly to TRAIL-R1, also TRAIL-2 signalling is independent of FADD (Pan et al., 1997b; Sheridan et al., 1997; MacFarlane et al., 1997). In contrast, Walczak et al., (1997) showed that overexpression of dominant-negative FADD strongly attenuated TRAIL-R2-mediated apoptosis in CV-1 cells. Testing BJAB cells expressing FADD-DN, we also observed a complete block of TRAILinduced signalling (unpublished data). Thus, whether FADD is directly or indirectly coupled to TRAIL-R2 signalling requires further investigation. It is possible that TRAIL-R2 recruits an unknown adaptor protein that triggers the caspase cascade or that, like TRADD for TNF-R1 and TRAMP, another molecule mediates binding of FADD to TRAIL-R2.


EFFECTOR CASPASES Caspase-8 (FLICE/MACH/Mch5) belongs to a growing family of proteases, which were recently named caspases (Alnemri et al., 1996). Caspases are aspartate-directed cysteine proteases that seem to be required for most apoptotic pathways. A detailed survey on the biochemistry of caspases will be provided in other chapters in this book. The following section briefly summarizes the involvement of individual caspases in death receptormediated apoptosis. First evidence for the involvement of caspases in CD95-mediated apoptosis came from pharmacological experiments that employed selective caspase inhibitors. YVAD-CMK, DEVD-CHO and similar peptides that mimic the P1 aspartate residue of caspase substrates, strongly suppressed CD95 and TNF-R1-mediated killing(Los et al., 1995; Enari et al., 1995; 1996; Miura et al., 1995; Hsu et al., 1995). Another clue came from studies with the poxvirus-derived CrmA protein, designated for cytokine response modifier A. CrmA encodes a highly specific serpin-like inhibitor of a number of caspases, including caspase-1 and caspase-8 (Ray et al., 1992; Komiyama et al., 1994; Pickup, 1994). Several groups have shown that over-expression of crmA efficiently suppressed CD95-mediated apoptosis in a variety of cell types(Los et al., 1995; Enari et al., 1995; Tewari and Dixit, 1995; Heinkelein et al., 1996). It was demonstrated that expression of microinjected or liposome-transfected CrmA protected cells from CD95-mediated killing. In addition, enzymatic measurements revealed that triggering of CD95 readily induces caspase proteolytic activity. Maximal caspase activity appeared within 15 to 20 min after CD95 engagement (Los et al., 1995). In contrast to Caenorhabditis elegans, so far more than ten mammalian members of the caspase family have been identified. Based on phylogenetic analysis they can be divided into three subfamilies. The ICE-like protease family includes ICE (caspase-1) (Thornberry et al., 1992; Ceretti et al., 1992),TX/ICH-2/ICErelII (caspase-4) (Faucheu et al., 1995; Munday et al., 1995; Kamens et al., 1995), TY/ICErelIII (caspase-5) (Munday et al., 1995; Faucheu et al., 1996) and ICH-3 (caspase-11) (Wang et al., 1996a). The CED-3-like family includes CPP32/YAMA/apopain (caspase-3) (Fernandes-Alnemri et al., 1994; Tewari et al., 1995; Nichohon et al., 1995), Mch2 (caspase-6) (Fernandes-Alnemri et al., 1996b), Mch3/ICELAP3/CMH-l (caspase-7) (Fernandes-Alnemri et al., 1995; Lippke et al., 1996; Duan et al., 1996), caspase-8 (FLICE/MACH/Mch5) (Muzio et al., 1996; Boldin et al., 1996; Fernandes-Alnemri et al., 1996a), Mch6/ICE-LAP6 (caspase-9) (Duan et al., 1996; Srinivasula et al., 1996b), and Mch4/FLICE2 (caspase-10) (Fernandes-Alnemri et al., 1996a; Vincenz and Dixit, 1997). The third subfamily consists only of Nedd2/ ICH-1 (caspase-2) (Wang et al., 1994; Kumar et al., 1994). All caspases are synthesized as zymogens that need to be activated by proteolytic cleavage. The active enzyme is composed of a heterotetrameric complex of two large subunits, containing the active center, and two small subunits, as can be deduced from the crystal structure of both caspase-1 and caspase-3 (Wilson et al., 1994; Walker et al., 1994; Mittl et al., 1997). Based on their structure and order of action in the death pathway caspases can be divided into initiators and executioners. It is known that at least for CD95-mediated apoptosis signalling is transmitted by sequential caspase activation. However, the exact order of caspase activation during execution of the death pathways is still obscure. A direct link between death receptor triggering and caspase activation was established by cloning of


caspase-8 as part of the CD95 DISC (Muzio et al., 1996). The proform of caspase-8 is recruited to the multimerized receptor and then likely activated by autoproteolytic cleavage at the DISC (Medema et al., 1997a). Thus, caspase-8 is regarded as the most upstream caspase in the CD95 pathway. As discussed previously, either caspase-8 itself or simultaneously a caspase-8-like caspase, such as caspase-10, may be involved in a similar fashion in the signal transduction of the other death receptors. It is assumed that an apical initiator caspase cleaves and activates downstream executioner caspases, though it is unknown which and how many caspases are needed for the final demise of the cell. Caspase-8 has been shown to directly cleave caspase-3, caspase-4, caspase-7, caspase-9 and caspase-10 in vitro, while caspase-2 and caspase-6 were cleaved indirectly by other caspase-8-activated caspases present in cellular extracts (Muzio et al., 1997). The order of other caspases in this cascade is not clear so far. Orth et al. (1996) place caspase-6 upstream of caspase-3 and caspase-7. It has also been demonstrated that caspase-3 can cleave and activate caspase-6, caspase-7 and caspase-9 (Fernandes-Alnemri et al., 1995,1996b; Srinivasula et al., 1996b). The reason for such a great variability of caspases in the mammalian system, in comparison to C. elegans, is at present unclear. So far, there is no report demonstrating that a single caspase is crucial for apoptosis signalling by death receptors. The most intensively studied caspase member is caspase-3, which is activated by multiple apoptotic signals including serum withdrawal, activation of death receptors, treatment with granzyme B, ionizing radiation, or staurosporine. Depletion of caspase-3 due to homologous recombination results in excessive accumulation of neuronal cells, due to a lack of apoptosis in the brain, whereas it has no effect in other tissues. This indicates that caspase-3 may be redundant in many cell types (Kuidz et al., 1996). As it is the case with other caspases, the role of caspase-1 in apoptosis is also controversial. It has been suggested that caspase-1 is involved in CD95-mediated apoptosis of thymocytes, in apoptosis of mammary cells following matrix loss, and in DNA damageinduced interferon regulatory factor-1 (JRF-1)-dependent lymphocyte apoptosis (Kuida et al., 1995; Boudreau et al., 1995; Tamura et al., 1995). However, others could not find an impairment of apoptosis in caspase-1(–/–) mice (Li et al., 1995b; Smith et al., 1997) or failed to demonstrate activation of caspase-1 upon CD95 triggering (Muzio et al., 1997). Therefore, either caspase-1 does not play a role in apoptosis signalling through the death receptors, or another caspase1-like caspase substitutes for its function in different cellular contexts. In addition to the growing number of caspases, different splice variants of numerous caspases have been reported (Wang et al., 1994; Alnemri et al., 1995, Femandes-Alnemri et al., 1995; 1996b; Boldin et al., 1996; Wang et al., 1996a; Vincenz and Dixit, 1997). Such splice variants were shown to either function as activators or inhibitors of caspase activation. Some of them might also represent nonfunctional protease species. Interestingly, most of the reported splice variants are only detected at the mRNA level and do not undergo translation. Therefore, the number of isoforms expressed as proteins is limited (Scaffidi et al., 1997). An increasing number of proteins have been found to be cleaved by caspases, yet the critical apoptosis-relevant substrates are still unknown (reviewed in Cohen, 1997; Nicholson and Thornberry, 1997). Sometimes cleavage results in the activation of a protein,


sometimes in its inactivation. Substrates include enzymes involved in genome function, such as the DNA repair enzyme poly(ADP-ribose) polymerase (PARP), DNA-PK, 70 kDa U1, heteronuclear ribonucleoproteins C, and the 140 kDa component of the DNA replication complex. Regulators of the cell-cycle progression including the retinoblastoma protein, the p53 regulator MDM-2 and protein kinase C-δ are also cleaved. Structural proteins of the nucleus and cytoskeleton that are cleaved by caspases include lamins, Gas2, gelsolin and fodrin, a non-erythroid spectrin. Furthermore, it has been found that endonucleolytic DNA cleavage is triggered upon caspase-mediated degradation of the 45 kDa subunit of DNA fragmentation factor (DFF) (Liu et al., 1997). One of the first death substrates found to be cleaved by caspases, particularly caspase-3 and -7, was PARP. Already before the discovery of caspases, PARP had been implicated in a variety of apoptotic events (Kaufmann et al., 1993). The enzyme catalyzes the transfer of ADP-ribose moieties from NAD to nuclear proteins which may result in protein modification and, following excessive PARP activation, NAD depletion. As DNA strand breaks activate the enzyme, PARP has been proposed to trigger DNA damage-induced apoptosis by depleting intracellular NAD stores. On the other hand, due to its DNA repair activity, PARP may exert a protective function. It has been proposed that during CD95induced apoptosis proteolytic cleavage of PARP inhibits most of its DNA repair activity, and thus may contribute to the demise of the cell (Tewari et al., 1995). To analyze whether PARP cleavage is a prerequisite for cell death, we have recently investigated CD95 and TNFR1-mediated apoptosis in PARP(–/–) mice. In a variety of cells and tissues, no significant differences between the apoptosis sensitivity of PARP(−/−) and parental mice were detected (Wang et al., 1997a; Los et al., unpublished results). Therefore, although PARP(–/–) mice have defects in maintaining genomic stability, cleavage of PARP is dispensable for death receptor signal transduction. Recently, cleavage of the p21-activated kinase, PAK2, during CD95 and TNFmediated apoptosis has been reported (Rudel and Bokoch, 1997). This cleavage results in a constitutively active kinase. Since PAK2 activates the stress-activated protein kinase pathway, it may provide the link between caspases and JNK/SAPK activation in cells undergoing apoptosis. Interestingly, blocking the activity of PAK2 by a dominant-negative mutant prevents the formation of apoptotic bodies during CD95-mediated apoptosis, whereas nuclear apoptosis as well as phosphatidylserine externalization remain unaffected (Rudel and Bokoch, 1997). This illustrates how different features of apoptosis might be discriminated at the level of caspase targets. A direct link between caspase-3 activation and DNA fragmentation was found by cloning of a heterodimeric factor, called DFF. It was demonstrated that DFF, when activated after cleavage of its 45 kDa subunit by caspase-3, induces DNA fragmentation on isolated nuclei (Liu et al., 1997). Therefore theoretically, a death receptor signalling pathway may involve only caspases, i.e. CD95 could activate caspase-8 that cleaves caspase-3 which in turn activates DFF. Despite compelling evidence for a key role of FADD-mediated recruitment of caspase-8 to CD95 and TNF-R1, it should be stressed that at present the precise scenario of receptormediated caspase activation is still fragmentary and most experiments have been only performed in a limited number of cell lines. The molecular modifications that control the recruitment process of caspases are not fully understood.


A novel molecular clue was provided by the cloning of Apaf-1 (apoptotic proteaseactivating factor-1), which represents a mammalian homologue of the C. elegans upstream death regulator protein Ced-4 (Zou et al., 1997). In C. elegans, Ced-4 physically interacts with both the Bcl-2 homologue Ced-9 and the caspase Ced-3, thus linking the upstream inhibitor and the downstream effector to a multicomponent death complex (Chinnaiyan et al., 1997; Wu et al., 1997). In addition, a direct binding between Ced-4 and mammalian caspase-1 and caspase-8, but not caspase-3 and caspase-8 has been observed. Apaf-1 is a 130 kDa protein that possesses three distinct domains. The C-terminal part of Apaf-1 is composed of 12 putative WD40 repeats, a motif known to mediate proteinprotein interactions. This region is followed by a stretch of 320 amino acids that are homologous to Ced-4. The N-terminal region of Apaf-1 shares sequence similarity with the N-terminal domain of Ced-3 and some other mammalian caspases. This domain serves as a so-called caspaserecruitment domain (CARD) by binding to caspases that have a similar CARD motif at their N-terminus (Hofmann et al., 1997). In particular, caspase-9 (Mch6, ICE-Lap6) is recruited to Apaf-1 (Li et al., 1997). In cells not undergoing apoptosis, the CARD is not exposed and therefore not bound to caspase-9. However, binding of ATP and cytochrome c, that is released from mitochondria during early cell death (Liu et al., 1996a), presumably induces a conformational change and unmasks the CARD in Apaf-1. This event finally culminates in the recruitment and activation of caspase-9 at the apoptosome complex. Similar to caspase-9, also caspase-1 and caspase-2 contain a CARD region. Thus, it is possible that these initiator caspases are recruited to Apaf-1 and may therefore act independently of FADD and other DED-containing proteins. This idea is supported by observations that some anticancer drugs induce caspase activation by a mechanism not requiring CD95/CD95L interaction (Fulda et al., 1997b, Wesselborg et al., submitted). In summary, further studies are necessary to unravel the caspase cascade induced by the different death receptors and to identify crucial targets for caspases that constitute the link between caspase activation and more downstream events in apoptosis. ALTERNATIVE DEATH SIGNALLING PATHWAYS Role of non-caspase proteases During apoptosis various nuclear and cytoplasmic proteins, which are not necessarily substrates for caspases, may be subjected to proteolytic breakdown. It is likely that caspases, once being activated, stimulate a cascade of other proteases resulting in the final demise of the cell. Indeed, the partially protective effect of certain protease inhibitors suggests that, in addition to caspases, other classes of proteases with different target specificity may contribute to the various morphological alterations of apoptosis (Chow et al., 1995; Schlegel et al., 1995). There are some hints that activation of caspases may be required but not sufficient to cause apoptosis in certain systems. Caspase activation may not necessary lead to apoptosis, as transient activation of caspase-3 during T-cell stimulation with PHA, which is not linked to apoptosis, can be observed (Miossec et al., 1997). Based on the protective effect of certain


inhibitors, an involvement of serine proteases has been reported in some forms of apoptosis. In U937 cells, a 24 kDa elastase-like serine protease, called AP24, has been purified that can induce the formation of a DNA ladder when added to isolated nuclei (Wright et al., 1994). The activity of the protease is rapidly activated during apoptosis triggered by UV irradiation and TNF treatment. Interestingly, there are some inhibitors of AP24 activation that do not affect caspase-3, but fully prevent DNA fragmentation and apoptosis (Wright et al., 1997). In addition, bFGF was found to indirectly prevent AP24 activation and cell death, but not PARP cleavage or caspase-3 activity in U937 cells. The involvement of serine proteases in death receptor pathways has been already suggested in early investigations on the mechanism of TNF-mediated cytotoxicity (Ruggiero et al., 1987; Suffys et al., 1988). In support of the participation of serine proteases in cell death execution, it was demonstrated that overexpression of plasminogen activator inhibitor-2 (PAI-2) prevented apoptosis in HT-180 and HeLa cells (Kumar and Bagglioni, 1991; Dickinson et al., 1995). Interestingly, in TNF-sensitive L929 cells overexpressing CD95, certain inhibitors of serine proteases such as TLCK abolish TNF-, but not CD95mediated cell death (Vercammen et al., 1997). As will be described in a later section, these observations indicate that in some cell types distinct effector molecules may participate in both pathways. Based on data obtained in a cloning approach to isolate positive regulators of apoptosis, cathepsin D, a lysosomal cysteine protease, was found and suggested to play a role in cell death mediated by IFNγ, TNF and CD95 (Deiss et al., 1996). In HeLa cells, cell death was inhibited following overexpression of an antisense cathepsin D construct or inactivation of the protease with the inhibitor pepstatin A. Since HeLa cells are normally completely protected against TNF-R1 and CD95-mediated cell death by caspase inhibitors, the relationship of cathepsin D and activation of caspases remains to be demonstrated. It should be noted that sometimes proteases are not involved in the execution but rather in the initial activation phase of apoptosis. For instance in T lymphocytes, inhibitors of proteasome function block T cell receptor (TCR)-mediated cell death but do not directly interfere with CD95-mediated apoptosis (Grimm et al., 1996a; Cui et al., 1997). It is established that TCR-mediated apoptosis is CD95-dependent and requires the inducible expression of CD95L (Dhein et al., 1995; Brunner et al., 1995; Ju et al., 1995). Some evidence suggests that CD95L expression in response to TCR ligation is controlled by a proteolytic step which involves the transcription factor NF-κB (Ivanow et al, 1997; Cui et al., 1997). Activation of NF-κB requires the proteolytic degradation of its inhibitory subunit IκB at the proteasome which allows active NF-κB to translocate into the nucleus (SchulzeOsthoff et al., 1995). We recently found that the proteasome-specific inhibitor PSI prevents TCR-induced cell death by inhibiting NF-κB-controlled CD95L expression (unpublished results). Thus, proteases may not only control the execution of cell death, but may be involved in signalling events required for the sensitization of cells to a specific apoptotic pathway. The sphingomyelin pathway Another apoptotic pathway implicated in death receptor-mediated apoptosis involves the generation of ceramide by the hydrolysis of the phospholipid sphin gomyelin. Ceramide is a


second messenger produced upon activation of sphingomyelinases (SMases) or via de novo synthesis by ceramide synthetase. Two forms of SMases can be distinguished based on their pH optima. Neutral SMase has a pH optimum of 7.4, requires Mg2+ ions and is found at the plasma membrane. Acidic SMase has the highest enzymatic activity at pH 5.0, is activated by diacylglycerol and mainly present in endosomes and lysosomes. A multitude of nonapoptotic and apoptotic stimuli can activate sphingomyelin turnover including ionizing irradiation, oxidative stress, treatment with doxorubicin or ligation of TNF-R1 and CD95 (Haimovitz-Friedman et al., 1994, Bose et al., 1995; Jaffrezou et al., 1996; Wiegmann et al., 1994; Cifone et al., 1995; Gulbins et al., 1995; Tepper et al., 1995). Ceramide generated as a result of sphingomyelin turnover in turn can stimulate various target molecules, such as ceramide-activated protein kinase (CAP kinase), ceramide-activated protein phosphatase (identical to PP2A), the protein kinase C isoform ζ, and Raf-1. A specific role for ceramide in mediating apoptotic signals was also suggested by the apoptotic effect of exogenous shortchain ceramides or the treatment of cells with bacterial SMase. TNF-R1 has been shown to activate neutral SMase through FAN (factor-associated neutral SMase), a protein that interacts with a stretch of 9 amino acids upstream of the DD. A dominant-negative mutant of FAN is able to block TNF-R1-mediated neutral SMase activation completely without affecting cell death (Adam-Klages et al., 1996). Recent data from two different groups, however, indicate that all functions exerted by TNF-R1 require a functional DD. First, overexpression of a trimerized TNF-R1 DD was sufficient to induce apoptosis and to activate NF-κB (Vandevoorde et al., 1997). Second, knock-out mice expressing a TNF-R1 transgene lacking the 30 terminal amino acids have the same phenotype as TNF-R1–/– mice. They are resistant to endotoxic shock and susceptible to Listeria infection (Pfeffer et al., 1993; T. Plitz and K. Pfeffer, personal communication). All these data suggest that the juxtamembrane region of TNF-R1 is not required for the main functions of the receptor. Hence, the relevance of FAN binding and activation of neutral SMase needs to be shown. Similar to TNF-R1, mutant CD95, which is defective in death signalling, is still able to activate neutral SMase (Cifone et al., 1995). Therefore, neutral SMase mediated ceramide production is independent of cell death signalling by CD95 and TNF-R1. Ceramide production by acidic SMase is mediated through the prior activation of the phosphatidylcholine-specific phospholipase C (PC-PLC). The region of TNF-R1 which initiates the PC-PLC/acidic SMase pathway corresponds to the DD of TNF-R1. The xanthogenate compound D609 inhibits this pathway and is able to prevent TNF-induced cell death in various cell types (Machleidt et al., 1996). However, it has been found that cells from patients with Niemann-Pick disease type A, which lack functional acidic SMase, are resistant to ionizing irradiation, but not to CD95- or TNF-R1-induced apoptosis (Santana et al., 1996). Therefore, although both neutral and acidic SMase have been implicated in ceramide production and death signalling through CD95 and TNF-R1, neither of them seems to be essential or sufficient for apoptosis induction by these receptors. Another metabolizing pathway for ceramide was recently proposed by Testi and coworkers to play a role in apoptosis (De Maria et al., 1997). Ceramide can be shuttled to the Golgi complex where it is converted to gangliosides. In myeloid and lymphoid cells it was found that CD95 ligation or treatment with ceramide resulted in the accumulation of the ganglioside GD3, an event, which was inhibited by caspase inhibitors. Moreover, antisense


oligonucleotides to GD3 synthetase, which resides in the Golgi complex, attenuated apoptosis, whereas overexpression of wild-type enzyme was associated with massive cell death. The authors suggested that during CD95-mediated apoptosis, GD3 ganglioside may be targeted to mitochondria where it alters mitochondrial function and causes cell death. It should be stressed that there is currently much confusion about the role of endogenous ceramide in apoptosis. Whereas some publications place ceramide production upstream of caspases, others suggest that it acts downstream of caspases, as it can be blocked by caspase inhibitors, such as CrmA, zVAD or DEVD (Dbaibo et al., 1997; Sillence and Allan, 1997; Gamen et al., 1996). Moreover, ceramide production may be not necessarily linked to apoptosis, as it is also observed after Ca2+ ionophore treatment (Sillence and Allan, 1997) without being associated with cell death. A possible reason for the discrepancy on the functional role of ceramides may lie in methodological problems. Ceramide production is generally determined in assays using bacterial DAG kinase. In a recent investigation in T lymphocytes, no ceramide production in response to CD95 ligation could be detected by mass spectroscopy, whereas under the same conditions an apparent increase of ceramides was measured by the classical DAG kinase assay (Watts et al., 1997). It was suggested that lysates from apoptotic cells may stimulate DAG kinase activity directly, which then may falsely reflect an increase in ceramide production. Thus, whether sphingomyelin hydrolysis is functionally involved in the propagation of death signals or rather represents a secondary modulatory pathway has to await careful reexamination. Stress-activated protein kinases A multitude of noxious stimuli lead to the activation of two related signalling pathways which center on two MAP kinase homologs, called the stress-activated protein kinase (SAPKs), also known as Jun N-terminal kinase JNK, and p38 which is the mammalian counterpart of yeast HOG1 (reviewed in Cahill et al., 1996a; Su and Karin, 1996). Known targets of these kinases include mostly transcription factors such as ATF-2, c-Jun or JunD which become activated after cell toxin exposure. Because many of the stress stimuli activating these kinases such as UV irradiation, heat shock and protein synthesis inhibitors, often also induce apoptosis, this hinted at the possibility that SAPK/JNK and p38 may be involved in the transmission of the death signal (Derijard et al., 1994; Hibi et al., 1993; Zanke et al., 1996; Meier et al., 1996). There are several reports linking apoptosis induced by CD95 and TNF-R1 ligation or other stress stimuli to the activation of SAPK/JNK and/or p38. Both CD95 and TNF-R1 are able to increase the activity of the kinases, although compared to many other stress stimuli activation by CD95 is usually delayed. Inhibition of p38 activity was found to be unable to prevent TNF-induced cell death of fibroblasts (Beyaert et al., 1996). Moreover, when the relationship between SAPK/JNK activation in response to TNF-R1 ligation, activation of NF-κB and induction of apoptosis was studied, TRAF2 and RIP were found to be involved in SAPK/JNK and NF-κB activation but not apoptosis, whereas dominantnegative FADD inhibited apoptosis but not kinase activation (Natoli et al., 1997; Liu et al., 1996b). Thus, dominant-negative TRAF2 and FADD mutants clearly dissociate SAPK/JNK activation from induction of apoptosis. The cytotoxic signal of TNF-R1, mediated through


TRADD/FADD, and the activation of SAPK/JNK, mediated through TRAF2 and RIP, are therefore two separate pathways that bifurcate at the level of receptor-associated molecules. CD95 can activate SAPK/JNK and p38, although TRAF2 is not associated with this receptor (Cahill et al., 1996b; Latinis et al., 1996; Goillot et al., 1997). SAPK/ JNK activation has been located downstream of caspases in the CD95 pathway, since it can be blocked by the caspase inhibitors zVAD and CrmA (Cahill et al., 1996b; Juo et al., 1997). In addition, SEK1, an upstream activator kinase of SAPK/ JNK, is able to inhibit SAPK/JNK activation when expressed as a dominant-negative mutant without affecting CD95-mediated apoptosis (Lenczowski et al., 1997). This again suggests that the pathway of SAPK/JNK activation is independent from apoptosis induction. At present, there are contradictory results placing SAPK/JNK activation right into the apoptotic pathway. A recent study identified a novel upstream activator kinase of the MAP kinase pathway, termed ASK1 (apoptosis signalling kinase-1) (Ichijo et al., 1997). The enzyme is assumed to contribute to TNF-mediated cytotoxicity, because a kinase-dead ASK-1 mutant inhibited TNF-induced apoptosis. Some studies further revealed that certain upstream elements of the different MAP kinase cascades are targeted and cleaved by caspases. As mentioned above, p21-activated kinase, PAK2, is cleaved during CD95 and TNFmediated apoptosis leading to a constitutively active kinase. Overexpression of a dominantnegative PAK2 mutant resulted in inhibition of the formation of apoptotic bodies, whereas other signs of apoptosis remained unaffected (Rudel and Bokoch, 1997). In addition, MAP kinase kinase-6b (MKK6b), an upstream mediator of p38 and SAPK/ JNK activation, was found to be activated in a caspase-dependent manner and to be necessary for CD95mediated apoptosis in Jurkat cells (Huang et al., 1997). In contrast, also a protecting effect of upstream components of MAP kinase cascades has been reported. Thymocytes deficient in SEK1 were found to be more sensitive towards CD95 and anti-CD3-induced apoptosis, whereas apoptosis induced by other environmental stresses was unaffected (Nishina et al., 1997). Finally, a direct link between death receptor signalling and activation of SAPK/JNK pathway was identified through the cloning of Daxx, a protein that interacts with the DD of CD95 and leads to caspase-independent SAPK/JNK activation. It was found that a dominant-negative SEK1 mutant was able to block both SAPK/JNK activation and cell death in certain cells (Yang et al., 1997a). Therefore, a secondary apoptotic pathway may exist in certain cells which is dependent on activation of MAP kinases and may cooperate with the caspase cascade. However, the relevance of this pathway in biological systems needs to be established. Reactive oxygen intermediates: necrosis versus apoptosis Cells die by one of the two mechanisms, necrosis or apoptosis, that can be distinguished by biochemical and morphological criteria (Farber, 1994). While triggering of a given death receptor will lead to apoptosis in most cells, there are some conditions and cell types where death receptors clearly mediate necrotic cell death. Necrosis is often referred to as accidental cell death and is induced when the plasma membrane of a cell is irreversibly damaged. Biochemically, these alterations seem to be less regulated than apoptosis, and a number of pathways have been implicated in necrosis, including generation of reactive oxygen intermediates (ROIs), activation of phospholipases, perturbation of calcium


homeostasis, and unspecific DNA and protein damage (reviewed in Beyaert and Fiers, 1994). In both necrosis and apoptosis, mitochondria play obviously a critical role, as in both forms of cell death a rapid and dramatic decrease in the mitochondrial membrane potential (∆Ψm) is observed (reviewed in Kroemer et al., 1997a). The drop in ∆Ψm is due to permeability transition and allows molecules to leak out from the mitochondrial matrix. The exact molecular events causing permeability transition are not known, but it is believed that the decrease in ∆Ψm represents the “point of no return” in a death pathway. In cells treated with apoptogenic agents, anti-apoptotic members of the Bcl-2 family that are localized at the mitochondrial membrane prevent permeability transition and the release of cytochrome c, thereby keeping the Ced-4 homologue Apaf-1 in an inactive state. During necrotic cell death, membrane permeability transition may lead to increased radical production, which in turn will cause cell damage through the oxidation of lipids, proteins and other components. This common occurrence of mitochondrial alterations, such as permeability transition, in necrosis and apoptosis indicates that some signalling processes might be shared between the two forms of cell death. Although it is not entirely clear which event decides whether a cell undergoes apoptosis or necrosis, the supply with ATP and other energy equivalents are likely determinants in the process (Tsujimoto, 1997; Leist et al., 1997). Among the death ligands, at least TNF has been reported to be able to induce apoptosis and necrosis (Laster et al., 1988). A necrotic cell death is exemplified for instance by TNFtreated L929 fibroblasts, which are often used as the prototype of TNF-sensitive cells. There is substantial evidence that during TNF-induced necrosis of fibroblasts mitochondria-derived ROIs are the critical mediators of cell death. Already early studies showed that TNF treatment caused ultrastructural abnormalities of mitochondria, as they appeared swollen and contained fewer cristae (Matthews and Neale, 1987; Schulze-Osthoff et al., 1992). Furthermore, when cells were treated with certain antioxidants or kept under anaerobic conditions, where no or less ROIs are produced, TNF cytotoxicity was strongly reduced (Schulze-Osthoff et al., 1992). Pharmacological experiments revealed that the mitochondrial respiratory chain was the major source of TNF-induced ROI formation. TNF cytotoxicity was strongly inhibited by rotenone and amytal, two drugs which inhibit the electron transfer at the level of complex I, thereby preventing ROI formation at the distally located ubiquinone (Schulze-Osthoff et al., 1992, 1993). In addition, it was observed that L929 cell clones, which had been selected for the depletion of mitochondrial DNA (mt-DNA) and therefore lacked mitochondrial respiration, were almost completely resistant to TNFinduced cytotoxicity (Schulze-Osthoff et al., 1993). Collectively, these evidences suggest that ROIs generated in the mitochondrial electron transport chain play an important role for TNF-induced necrosis. Interestingly, cells that are devoid of mt-DNA and hence of a functional respiratory chain can still undergo apoptosis, for instance induced by staurosporine treatment or CD95 ligation (Jacobson et al., 1993). This can be explained by the fact that mt-DNA-deficient cells still maintain a mitochondrial membrane potential and thus may release cytochrome c or other factors that can engage the apoptotic machinery. Thus, although in some cells TNFR1 and CD95-mediated cell death appears to involve similar signalling pathways, there are also examples demonstrating that death induction by the two death receptors must be different.


Differences between TNF-R1 and CD95 signalling are sometimes evident even within the same cell. L929 fibroblasts overexpressing human CD95 exhibit typical alterations of apoptosis when stimulated with anti-CD95, such as membrane blebbing, cytoplasmic shrinkage and internucleosomal DNA fragmentation. In contrast, in the same cells, TNF induces necrosis as evident by changes in the mitochondrial ultrastructure and lack of nuclear apoptotic alterations (Schulze-Osthoff et al., 1994; Vercammen et al., 1997). Differences between both receptors are also observed when the effects of pharmacological inhibitors are analyzed. While mitochondrial inhibitors or antioxidants almost completely block TNF-R1-mediated cell death, inhibition of ROI production does not affect CD95mediated cytotoxicity (Schulze-Osthoff et al., 1994; Hug et al., 1994). The different drug effects may be related to the distinct morphological forms of cell death that are induced upon CD95 and TNF-R1 in certain cell types. There are other differences between TNF-R1 and CD95-mediated signal transduction. As described above, TNF is a potent inducer of transcription factor NF-κB and proinflammatory gene expression, whereas the biological function of CD95 is largely restricted to apoptosis. As ROIs have been proposed as key second messengers of NF-κB activation (reviewed in Schulze-Osthoff et al., 1995), the lack of NF-κB activation by CD95 concurs with the notion that CD95 signal transduction is ROIindependent. Another difference between CD95 and TNF-R1 relies in the time course of killing induced by both death receptors. In most cell lines, CD95 triggers death very rapidly, in line with the instant recruitment of caspase-8 to the death receptor complex. In contrast, TNF-R1-mediated cell death proceeds slower, as there is mostly a delay of several hours between receptor ligation and the initial signs of cell death. The reasons for the different kinetics are unknown, but they are inconsistent with the idea of similar death inducing complexes of the two receptors. It has been further observed that a number of cell lines are only sensitive to either CD95 or TNF-R1, although both receptors are expressed at similar amounts (Wong and Goeddel, 1994; Grell et al., 1994). While ROIs may be selectively involved in death pathways leading to necrosis, caspases may be restricted to apoptotic alterations of cell death. Under certain circumstances, such as depletion of ATP by the drug oligomycin, apoptosis can be shifted to necrosis (Leist et al., 1997). Furthermore, in some cells, caspase inhibitors potently block the appearance of apoptotic alterations, but the final cell death is not stopped. For instance, we recently observed that in the presence of caspase inhibitors CD95 ligation induces necrosis instead of apoptosis in some fibroblast cell lines (Los et al., submitted). This may be explained by the fact that under these conditions cytochrome c is still released, ∆Ψm drops, ROIs are produced, and consequently cells die by a caspase-independent mechanism. It should be noted that these findings may also constrain the potential therapeutic use of caspase inhibitors under certain conditions of cell death. CELLULAR ANTI-APOPTOTIC MECHANISMS Whether triggering of a death pathway results in cell death, not only depends on the expression level of a death receptor and effector molecule, but also on resistance mechanisms that counteract an apoptogenic signal. Often apoptosis is enhanced by inhibitors of protein synthesis indicating that cells produce shortlived proteins that would normally prevent cell


death. Thus, the balance between destructive apoptotic signals and protective mechanisms determines the outcome of a given death stimulus. Among many other functions, apoptosis serves as a important defense mechanism of the organism to combat viral infections and to prevent virus spreading (Collins, 1995; Teodoro and Branton, 1997). It is therefore not suprising to find that viruses have developed their own or adopted the host’s mechanisms to suppress apoptosis. The identification of viral anti-apoptotic genes led in many cases to the subsequent discovery of their cellular homologues that act to prevent cell death. In the following section, we will describe various mechanisms that interfere with cell death at very distinct steps in the signal transduction pathway of death receptors. Receptor-associated mechanisms The most proximal step to suppress a death receptor pathway is of course the inhibition of ligand binding. Members of the death receptor family can sometimes be found as truncated, soluble forms of the extracellular domain, which are either derived from alternative gene splicing or from proteolytic shedding of the receptor molecule. It has been proposed that expression of the soluble extracellular part of CD95 is enhanced and may account for defective apoptosis and development of systemic lupus erythematosus (SLE) (Cheng et al., 1994), though this finding could not be confirmed by other investigators (Mysler et al., 1994; Knipping et al., 1995). A very interesting mechanism of neutralizing a death ligand is found in the TRAIL system. The membrane expression of the specialized decoy receptors, DcR1 and DcR2, which binds TRAIL without signalling for cell death, is held responsible for the resistance of normal cells to TRAIL cytotoxicity (Pan et al., 1997b; Sheridan et al., 1997; Degli-Eposti et al., 1997; MacFarlane et al., 1997). A further downstream level, at which induction of apoptosis can be prevented, is the signalling activity of a death receptor. For CD95, a negative regulatory role for the Cterminus of the receptor has been suggested, since deletion of its last 15 amino acids increases the sensitivity towards CD95-induced apoptosis (Itoh and Nagata, 1993). By interaction cloning and in vitro binding studies, this region of CD95 has been found to interact with a protein tyrosine phosphatase, called Fas-associated phosphatase-1 (FAP-1) (Sato et al., 1995). Overexpression of FAP-1 partially inhibits CD95-induced apoptosis, while its expression inversely correlates with the sensitivity in T helper cell subsets (Zhang et al., 1997). The region of CD95 that is required for interaction with FAP-1 has recently been narrowed down to the last 3 amino acids, and microinjection of this tripeptide into cells, which blocks binding of FAP-1 to CD95 in vitro, facilitates CD95 signalling (Yanagisawa et al., 1997). These results are consistent with a negative regulatory role for FAP-1 in CD95 signalling. Yet, so far conclusions are mainly based on correlations and only an association of FAP-1 with human but not mouse CD95 has been detected (Cuppen et al., 1997). Together with the markedly different expression patterns of FAP-1 and CD95, it is unlikely that FAP-1 plays a key role in CD95 signal transduction. Recently, it was suggested that signal transduction of TNF-R1 and CD95 may be modulated by a pathway related to ubiquitination. A novel protein, called sentrin, was isolated in a two-hybrid screen and found to bind to the DD of CD95 and TNF-R1 (Okura et al., 1996). Sentrin exhibits homology to ubiquitin but itself does not contain a DD. When overexpressed, sentrin provides protection against TNF and CD95-mediated apoptosis.


Sentrin associates strongly with the ubiquitinconjugating enzyme UBC9 (Gong et al., 1997) that was found to be also associated with CD95 (Becker et al., 1997). It is believed that UBC9 catalyzes the conjugation of ubiquitin or sentrin to other molecules. Whether this process, called sentrinization, inhibits cell death by blocking binding of DD adaptor proteins or other molecules remains to be elucidated. FLICE (caspase-8) inhibitory proteins When it became evident that caspase-8 contains two regions at its N-terminus that shared sequence homology with the DED of FADD, this information was used to search for other regulatory proteins containing DED motifs. In screening EST databases, a family of proteins encoded by different viral genomes with homology to the DEDs of FADD and caspase-8 was identified. Members of this family are the E8 protein of equine herpesvirus 2, the ORF71 of human herpesvirus 8 and herpesvirus saimiri, a protein encoded by the bovine herpesvirus 4 and two genes (ORF159L and ORF160L) of human moluscipoxvirus (reviewed by Peter et al., 1997a). These proteins were called v-FLIPs (viral FLICE inhibitory proteins), because, following overexpression in mammalian cells, they bind to the death receptor-FADD complex and prevent caspase-8 recruitment and DISC formation (Thome et al., 1997; Hu et al., 1997a; Bertin et al., 1997). v-FLIPs have a unique structure, as they consist of two DEDs without bearing a caspase-like catalytic domain. v-FLIPS can thereby act as dominantnegative proteins and compete for caspase-8 recruitment to the DISC. Consequently, overexpression of v-FLIPs results in protection against CD95-, TNF-R1-, and TRAIL receptor-induced apoptosis (Thome et al., 1997; Hu et al., 1997a; Bertin et al., 1997). vFLIPs are the first known anti-apoptotic viral proteins that interfere with the most proximal signalling events of death receptors. For the herpesvirus saimiri (ORF71)-FLIP, it was shown that the protein is expressed late in the lytic cycle and thereby renders the cells resistant to CD95. Therefore, v-FLIPs may protect infected cells from premature apoptosis induced by viral overload (Thome et al., 1997). Shortly after the discovery of v-FLIPs, a cellular homologue, called c-FLIP, was found that presumably acts as a principal death regulator in mammalian cells (Irmler et al., 1997). cFLIP was independently identified by several groups and is also termed Casper, I-FLICE, FLAME-1, CASH, CLARP, and MRIT (Shu et al., 1997; Hu et al., 1997b, Srinivasula et al., 1997, Goltsev et al., 1997; Inohara et al., 1997; Han et al., 1997; reviewed in Wallach, 1997). Its gene was localized to the long arm of chromosome 2 (2q33–34) where also the caspase-8 and caspase-10 genes are clustered. c-FLIP is expressed in a variety of tissues and occurs in two splice variants. Similar to v-FLIP, the short form c-FLIPS contains two DED motifs at the N-terminus and through them it can bind to other DED-containing proteins. The long form FLIPL, in addition, contains at its C-terminus a region that resembles the proteolytic region in caspase-8 and -10. The most remarkable feature of c-FLIP is that the active-site cysteine is absent and substituted by a tyrosine residue. Therefore c-FLIP can be suggested to be proteolytically inactive. It was found that c-FLIP can associate with different proteins including FADD, caspase-8 and caspase-3. By some investigators, also an independent association with the TNF-R1bound protein TRAF2 and with Bcl-xL was noted (Shu et al., 1997; Han et al., 1997). Transfection of c-FLIP cDNA into mammalian cells had profound effects on cell death.


Curiously, quite opposite effects were reported by different groups. Mostly, a protective effect of c-FLIP on TNF-R1, CD95, TRAMP or TRAIL-induced apoptosis was detected (Irmler et al., 1997; Hu et al., 1997; Srinivasula et al., 1997). This concurs with observations made for the viral homologues that FLIPs act as dominant-negative inhibitors that prevent caspase-8 activation. In contrast, other studies reported a marked cytotoxic effect of c-FLIP which was inhibitable by caspase inhibitors (Shu et al., 1997; Han et al., 1997b; Inohara et al., 1997). The pro-apoptotic effect could result from the ability of c-FLIP to activate caspase-8 via homophilic interaction through its corresponding DED. This mechanism would be similar to that previously suggested for the FADD/caspase-8 interaction. Alternatively, since c-FLIP can associate with TRAF2, it was suggested that c-FLIP overexpression might displace the anti-apoptotic proteins c-IAP1 and C-IAP2 from the TNFR2/TRAF2 complex (Shu et al., 1997). This discrepancy in the observed effects may be also due to the cell line used, and the artificial system of the transfection experiments. It underscores that in vitro effects may not necessarily correspond to the function of a protein when expressed in the physiological range within the organism. Because a pro-apoptotic effect of c-FLIP was seen especially following massive overexpression of the protein, the true function of c-FLIP may be restricted to the inhibition of apoptosis. This view also correlates with the amount of c-FLIP expressed in T lymphocytes. c-FLIP is largely expressed during the early stage of T cell activation. In these cells it was shown that caspase-8 is not recruited to the CD95/FADD complex (Peter et al., 1997b). c-FLIP disappears when T cells become susceptible to CD95-induced apoptosis and a functional CD95 DISC is formed. High levels of c-FLIP were also found in melanoma cells and malignant melanoma tumors (Irmler et al., 1997). The family of IAPs Inhibitor of apoptosis proteins (IAPs) constitute a family of molecules that are conserved throughout evolution and prevent cell death in several systems. Originally two IAPs, CpIAP and Op-IAP, were discovered in baculovirus and found to functionally complement the death inhibitor p35 (Crook et al., 1993; Birnbaum et al., 1994). Recently, Drosophila IAPlike proteins, designated DIAP1 and DIAP2 (DILP/DIAP) were cloned that inhibit cell death in insects (Hayand et al; 1995; Liston et al., 1996; Duckett et al., 1996). The first human IAP to be identified was the neuronal apoptosis inhibitory protein (NIAP) which was isolated based on its contribution to the neurodegenerative disorder, spinal muscular atrophy (Roy et al., 1995). Subsequently, four other human IAPs, called c-IAP1, C-IAP2, XIAP and survivin, have been isolated and demonstrated to counteract cell death (Rothe et al., 1995; Duckett et al., 1996; Liston et al., 1996; Ambrosini et al., 1997). The c-IAP1 and CIAP2 proteins were originally identified as molecules that are recruited to the cytosolic domain of TNF-R2 via their association with TRAF1 and TRAF2 (Rothe et al., 1995). cIAP1, in addition, has been shown to be a component of the TNF-R1 complex via its association with TRAF2 (Shu et al., 1996). The common structural feature of all IAP family members is a Cys/His-rich motif termed the baculovirus IAP repeat (BIR) that is present in one, two or three copies. With the exception of NIAP and survivin, all other IAP family members also contain a zinc finger-like RING domain at their carboxy terminus. The fact that the BIR motif is shared by all


members suggest a central role for this domain in mediating cellular protection. The function of the RING domain, however, is elusive. Recently, it was shown that some IAP members, including X-IAP, c-IAP1 and C-IAP2, can bind to and potently inhibit certain caspases, such as caspase-3 and caspase-7 (Deveraux et al., 1997; Roy et al., 1997). Inhibition of these caspases was also found in in vitro assays, whereas the activity of the proximal proteases caspase-1, caspase-6 and caspase-8 was not affected. Mutational analysis revealed that the BIR domains were sufficient for the inhibitory effect, though proteins that retained the RING finger domain were more efficient. In contrast to the other human IAPs, NIAP did neither bind nor inhibit caspases (Roy et al., 1997), suggesting that this and perhaps other IAP members may additionally have alternative targets of apoptosis inhibition. A detailed analysis of the mechanism of inhibition revealed that IAPs bind but are not cleaved by the caspases (Roy et al., 1997). This is in contrast to the poxvirus inhibitor CrmA and baculovirus protein p35 that are suicide inactivators which require peptide bond hydrolysis as part of their inhibitory mechanism (Bertin et al., 1996; Xue and Horvitz, 1995; Bump et al., 1995; Komiyama et al., 1994). At present, the role of c-IAP1/2 recruitment to the TNF receptors is unknown. Inhibition of caspases is independent of TNF-R binding function (Roy et al., 1997), though it is possible that recruitment of IAPs to the TNF-R may promote their interaction with caspases. Alternatively, it has been also suggested that the function of C-IAP2 may be associated with the activation of NF-κB, which is known to play a role of an anti-apoptotic transcription factor (see below). Overexpression of C-IAP2 directly stimulated NF-κB activation and interfered with TNF-induced cell death (Chu et al., 1997). This effect required the RING domain of C-IAP2 indicating that, in addition to caspase inhibition, IAPs may prevent apoptosis by alternative mechanisms. The Bcl-2 family of proteins Bcl-2-related proteins constitute another important decisional point of cell death. The Bcl-2 family consists of two functional classes of proteins including anti-apoptotic members, such as Bcl-2, Bcl-xL, Mcl-1, Bcl-w, Bfl-1, Brag-1 and A1, as well as proapoptotic molecules, such as Bax, Bad, Bak, Bid, Bik and Hrk (reviewed by Otvai and Korsmeyer, 1994; Reed, 1997; Kroemer, 1997b). Members of the Bcl-2 family are characterized by up to four conserved regions, termed the Bcl-2 homology (BH) domains. Many of these proteins interact with each other through a complex network of homo- and heterodimers. Originally, it has been proposed that the relative ratio of pro- and anti-apoptotic members dictates whether a cell will respond to a proximal apoptotic stimulus or not. Indeed, overexpression of Bcl-2/Bcl-xL abrogates cell death induced by a multitude of apoptotic stimuli (Cory, 1995, Boise et al., 1993), while enforced expression of Bax promotes apoptosis (Oltvai et al., 1993). Recent evidence, however, has challenged and at least partially invalidated the model of heterodimerization of Bcl-2 proteins. It has been proposed that heterodimerization may be due to the presence of non-ionic detergents in the dimerization assays and that the family members might not interact in vivo (Hsu et al., 1997b). In accordance, selected Bcl-xL mutants which fail to heterodimerize with Bax can still inhibit apoptosis (Cheng et al., 1996; Clair et al., 1997). Vice versa, mutants of Bax and Bak which do not bind Bcl-xL still exert


their pro-apoptotic activity (Simonian et al., 1996, 1997). It therefore seems that these Bcl-2 family members instead of heterodimerizing compete for a common binding partner. A variety of models have been proposed to explain how Bcl-2-related proteins inhibit apoptosis. Bcl-2 seems do prevent all alterations of apoptosis that occur at the level of mitochondria. Thus, Bcl-2-inhibitory proteins have been shown to prevent the formation of mitochondrial ROIs, alterations in calcium homeostasis, and mitochondrial membrane permeability transition. Interesting findings also came from cell-free experiments. In vitro, Bcl-2 prevents the release of an apoptogenic protease, called apoptosis-inducing factor (AIF), by isolated mitochondria (Susin et al., 1996). In addition, Bcl-2 blocks the redistribution of cytochrome c which released in early stages of apoptosis (Kluck et al., 1997; Yang et al., 1997b). These observations therefore suggest that the outer mitochondrial membrane is the principal site of action of Bcl-2. Recent studies demonstrated that after transient transfection Bcl-xL can bind to caspase-8, suggesting that this interaction would inhibit activation of caspase-8 (Chinnaiyan et al., 1997). In contrast, in MCF7-Fas cells, which were selected for CD95-resistance by overexpression of Bcl-xL, no association between caspase-8 and Bcl-xL was detected (Medema et al., 1997b). Likewise, activation of caspase-8 in response to CD95 ligation was unaffected. However, although DISG-bound caspase-8 may not be targeted by Bcl-2, the processing of downstream caspases including caspase-3 is prevented by Bcl-2 or Bcl-xL in a number of apoptotic systems (Monney et al., 1996; Boulakia et al., 1996). Thus Bcl-2related proteins may inhibit cell death by acting downstream of initiator but upstream of executioner caspases. Experiments in vitro demonstrated that Bcl-2 can interact with Apaf-1, a mammalian Ced-4 homologue. It is possible that Bcl-2 thereby sequesters the Apaf-1 complex at the outer mitochondrial membrane. Studies with isolated mitochondria revealed that Bcl-2 inhibits the release of the mitochondrial protease AIF and cytochrome c which otherwise, upon binding to Apaf-1, would activate the caspase cascade (Kluck et al., 1997; Yangr et al., 1997a; Susin et al., 1996). An interesting clue to the mechanism of action of Bcl-2 proteins may be provided by the three-dimensional structure of Bcl-xL. Its X-ray and NMR structure reveals a similarity to the pore-forming domains of bacterial toxins such as diphteria toxin and colicins (Muchmore et al., 1996). Indeed, both Bcl-2 and Bcl-xL have been shown to form channels in artificial lipid bilayers with a selectivity for K+ (Schendel et al., 1997; Minn et al., 1997). Different Bcl-2 family members might form channels in mitochondria with distinct selectivity which could regulate permeability of the outer mitochondrial membrane. In this context, it has been shown that Bcl-2 inhibits mitochondrial swelling during apoptosis that is presumably caused by a damage of the outer mitochondrial membrane (Vander Heiden et al., 1997). This osmotic swelling precedes redistribution of cytochrome c and membrane permeability transition. It is still very controversial whether Bcl-2 affects apoptosis induced by CD95 or other death receptors. From several reports it appears that there are certain cell types where Bcl-2 or related proteins provide potent protection against CD95-induced apoptosis (Itoh et al., 1993; Mandal et al., 1996), while there are other cells in which Bcl-2 is completely ineffective (Vanhaesebroeck et al., 1993; Memon et al., 1995). Since CD95-mediated caspase-8 activation at the DISC level is not inhibited by Bcl-xL, execution of apoptosis could be independent of the release of cytochrome c and other pro-apoptotic mitochondrial


components. In contrast, Bcl-2 may inhibit cell death in such cells, where execution of apoptosis depends on Apaf-1 and cytochrome c-induced activation of executioner caspases. It will be interesting to investigate, how the FADD/caspase-8 pathway is connected to mitochondria and whether caspase-8 can directly cause mitochondrial swelling, for instance by cleavage of proteins at the outer mitochondrial membrane. Important progress has been obtained how extracellular signals prevent cell death at the level of Bcl-2 proteins. Survival factors such as the cytokine IL-3 induce the phosphorylation of the pro-apoptotic protein Bad (Zha et al., 1996). This phosphorylation of Bad occurs at two serine residues which are embedded in the binding site of the phosphoserine-binding protein 14–3–3. As a consequence, phosphorylated Bad is bound to 14–3–3 and retained in the cytosol, but cannot interact with Bcl-xL and exert its pro-apoptotic function at the mitochondrial membrane. Two protein kinases, Raf-1 and Akt/PKB, have been implicated in phosphorylating Bad. Sequence comparison of Bcl-2 and other anti-apoptotic members reveals a domain (BH4) which is not present in the pro-apoptotic members. This domain interacts with Raf-1 and thereby targets the kinase to outer mitochondrial membrane (Wang et al., 1996b). Raf-1 in turn can phosphorylate the pro-apoptotic protein Bad. Raf-1mediated phosphorylation of Bad prevents Bad from binding to Bcl-2 and Bcl-xL, thus relieving repression of these antiapoptotic proteins by allowing them to homodimerize with themselves. In addition, there is strong evidence that the phosphoinositide-3 kinase (PI3K) pathway is the essential component in growth factor-mediated protection in vivo. The lipids produced by PI3K bind to and activate the serine/threonine kinase Akt/PKB. Activated forms of Akt/PKB have been shown to protect cells from apoptosis, for instance induced by growth factor withdrawal or by detachment of adherent cells from their extracellular matrix (anoikis) (reviewed in Marte and Downward 1997). It has been demonstrated that Akt/PKB phosphorylates Bad (Datta et al., 1997; del Peso et al., 1997). Upon phosphorylation, Bad dissociates from Bcl-xL which is then free to resume its activity as a suppressor of cell death. Very recently, it has been reported that CD95-mediated apoptosis is inhibited by overexpression of Akt/PKB (Häusler et al., 1998). Other anti-apoptotic mechanisms Cells contain a variety of other mechanisms providing protection against cell death. Heat shock proteins (hsps) constitute a group of proteins that based on their molecular size are divided into small hsps (shsp), hsp60, hsp70, and hsp90 subfamilies (reviewed in Jäättelä and Wissing, 1992). All hsps are rapidly induced by heat shock treatment and other stress stimuli. Their principal function seems to be to act as molecular chaperones in the process of protein folding. Overexpression of hsps was shown to enhance survival of cells exposed to numerous injuries that lead to death including heat shock, oxidative stress, treatment with anti-cancer drugs and other apoptosis-inducing agents. However, the molecular mechanism how hsps protect against apoptosis, is largely undefined at present. It has been proposed that hsps may counteract stress-induced disruption of the microfilament network which often occurs during apoptosis (Lavoie et al., 1993). Overexpression of hsp70 protects against TNFinduced apoptosis (Jäättelä, 1993). Whether this effect is associated with the attenuation of TNF-induced phospholipase A2 activation remains to be shown. Likewise, overexpression of shsps confers enhanced resistance to apoptosis induced by oxidative stress, TNF, CD95,


staurosporine and anticancer drugs (Mehlen et al., 1996a; 1996b). It has been hypothesized that shsps protect cells by inhibiting the action of ROIs. Though no in vitro detoxificant activity was found to be associated with shsps, overexpression led to an increased cellular content of the antioxidant glutathione and decrease in lipid peroxidation. The Zn-finger protein A20, another anti-apoptotic protein, was first identified by differential screening of a cDNA library from TNF-activated endothelial cells (Opipari et al., 1992). Gene expression of A20 is tightly regulated by transcription factor NF-κB that binds at two κB-binding sites within the A20 promoter (Krikos et al., 1992; Laherty et al., 1992). Overexpression of A20 strongly attenuates TNFinduced NF-κB activation and cytotoxicity. However, the ability to protect against apoptosis seems to be largely restricted to the kind of apoptotic stimulus. While protection against TNF-induced apoptosis has been shown in several cell types, no protection could be demonstrated against cell death triggered by serum depletion, oxidative stress or CD95 (Opipari et al., 1992; Jäättelä et al., 1996). It is likely that this selectivity is conferred by the specific molecular mechanism of A20 action. Though the biochemical mechanism of action of A20 is unknown, it has been found that A20 interacts with the TNF-receptor associated proteins TRAF1 and TRAF2 (Song et al., 1996). How A20 inhibits apoptosis, which, as mentioned above, is not mediated by TRAF1/TRAF2 but by TRADD, remains to be determined. NF-κB as an anti-apoptotic transcription factor Many cell types are resistant to apoptosis induced by death receptors, but become sensitive in the presence of inhibitors of RNA and protein synthesis. This observation can be explained by the fact that triggering of a death receptor not only mounts an apoptotic response, but simultaneously initiates a genetic programme that serves to block cell death and so sets up a delicate life-death balance. Transcription factor NF-κB, which was previously known mainly as a central mediator of inflammatory gene expression, has been recently implicated in protecting cells against apoptosis, most likely by inducing the expression of anti-apoptotic genes (Figure 1.3) (reviewed in Baichwal and Baeuerle, 1997). NF-κB consists of a heterodimeric complex which is often composed of the p50 and the p65 kDa RelA subunit (reviewed in Baeuerle and Baltimore, 1996). In its inactive state, the complex is sequestered in the cytosol by a third inhibitory subunit, called IκB. Upon stimulation of cells with TNF or a variety of other inflammatory stimuli, IκB is phosphorylated at its N-terminal serine residues and rapidly degraded at the proteasome. This event in turn releases the active form of NF-κB and allows for nuclear translocation, subsequent DNA binding and activation of NF-κB target genes. A first hint for a connection between NF-κB and cell death came from studies on knockout mice that lacked the RelA subunit of NF-κB (Beg et al., 1995). These mice died before birth and showed massive degeneration of liver cells caused by apoptosis. It was further demonstrated that fibroblasts and macrophages from RelA-deficient mice are hypersensitive to TNF-induced cytotoxicity, whereas wildtype cells survive this treatment (Beg and Baltimore, 1996). The susceptibility of RelA-deficient cells was reversed following transfection of cells with the wild-type relA gene. In another approach, it was shown that cells expressing a dominant-negative mutant of IκB were sensitized to TNF and even killed in the absence of protein synthesis inhibitors


Figure 1.3 Model of TNF-R1 induced signal transduction. TNF-R1 ligation results in two major cellular responses, activation of transcription factor NF-κB and induction of cell death. NFκB activation by TNF-R1 involves recruitment of the adapter proteins TRAF-2 and RIP. This event finally results in activation of the multisubunit IκB kinase which phosphorylates the NF-κB inhibitor IκB. Phosphorylated IκB dissociates from the inactive NF-κB heterodimer and allows NF-κB to translocate to the nucleus and to activate gene expression of survival genes. The cytotoxic response of TNF-R1 ligation is mediated by the DDcontaining protein TRADD which can interact with FADD and therefore allow for a potential crosstalk between the TNF-R1 and CD95 death receptor pathways.

(Wang et al., 1996c; van Antwerp et al., 1996). Conversely, pretreatment of cells with IL-1β, a cytokine that activates NF-κB but is not pro-apoptotic, protected cells from TNFinduced apoptosis. An increased resistance conferred by NF-κB was also observed with proapoptotic stimuli other than TNF. The chemotherapeutic drug daunorubicin and ionizing irradiation which also induce NFκB are more toxic, when NF-κB activation was blocked by a dominant-negative IκB mutant. In addition, a role of NF-κB in preventing apoptosis is evident in B-lymphocyte cell lines. Such cell lines express constitutively NF-κB, but inactivation of NF-κB by various means induces apoptosis (Wu et al., 1996). Thus, in B cells constitutive NF-κB has a role in ensuring cell survival. A general role for NF-κB as a transcription factor that prevents cell death is, however, far from being established. Apoptosis triggered by CD95 is not countered by NF-κB (van


Antwerp et al., 1996; unpublished results), presumably because CD95 does not lead to detectable NF-κB activation in most cell types. This may be due to the rapid induction of cell death, as NF-κB activation by CD95 can be restored when apoptosis is inhibited (Ponton et al., 1996). In addition, there are some reports pointing to a pro-apoptotic role of NF-κB. For instance, glutamateinduced toxicity in neurons has been found to be accompanied by NF-κB induction (Grilli et al., 1996). In this case, NF-κB seems to cause cell death, as blocking its activation with aspirin and sodium salicylate protected cells from excitotoxic damage. In another study, apoptotic death induced by serum withdrawal was demonstrated to induce NF-κB activation which was prevented by overexpression of Bcl-2 (Grimm et al., 1996b). Furthermore, Sindbis virus-induced apoptosis in a carcinoma cell line was shown to require NF-κB activation, as antisense NF-κB oligonucleotides prevented this type of cell death (Lin et al., 1995). The putative anti-apoptotic genes that are activated by NF-κB remain to be identified. Likely candidates are the genes encoding for manganese superoxide dismutase (MnSOD), CIAP2 and the zinc finger protein A20. Expression of all three genes is induced by TNF. MnSOD, an enzyme that converts superoxide anion to hydrogen peroxide inside mitochondria, may be involved by providing protection against TNF cytotoxicity, in particular necrosis which is largely dependent on ROI formation (Wong et al., 1989). CIAP2 is an NF-κB-controlled target gene product, which, as described in a previous section, may inhibit apoptosis at the level of either caspases or proximal TNF receptor signal transduction. Also A20 expression is tightly regulated by NF-κB and provides protection against TNF in number of cell types (Opipari et al., 1992; Krikos et al., 1992). Yet, transfection of A20 in RelA(–/–) 3T3 cells does not rescue these cells from TNF-induced apoptosis (Beg and Baltimore, 1996), suggesting that other resistance genes must be also involved. Certainly, the function of NF-κB as a regulatory loop that operates to provide protection against apoptosis, has important implications for tumor therapy. If tumor cell sensitivity is controlled by NF-κB, a combination of NF-κB inhibitors and chemotherapeutic drugs may result in improved efficacy of anticancer therapy. CONCLUSIONS This review describes the different death receptors and the pathways that are used either to induce or to counteract apoptosis. Certainly, the rapid and very recent discovery of a great variety of different death receptors with their multitude of control points to either execute or to inhibit cell death proceeds well ahead of our understanding of their role in multicellular systems. Our present view of the different functions of the various death systems is still fairly restricted and it can be anticipated that more death receptors and elements of their signalling pathways will be identified. A major objective of future research will be to study the biology of death systems in the whole organism and to address questions such as why there are so many different death systems, when and how do they operate and what ensures their specificity. The other major challenge is to apply the knowledge of death receptor networks to therapy. Therapeutic targeting of certain death pathways would be beneficial for very diverse diseases including AIDS, hepatitis, neurodegeneration, multiple sclerosis, stroke, myocardial ischemia and others. While the goal of treating acute degenerative diseases is to prevent unwanted cell death, the major challenge in treating cancer is to kill


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2. THE ROLE OF SPHINGOLIPIDS IN STRESS RESPONSES AND APOPTOSIS IN EUKARYOTES SHEREE D.LONG* AND YUSUF A.HANNUN*† *Department of Medicine, Duke University Medical Center, Durham, North Carolina 27710

Sphingolipids constitute an important class of membrane lipids in eukaryotic cells whose structural complexity exceeds that of phospholipids. With the elucidation of the sphingomyelin cycle, in which intracellular ceramide is generated, sphingolipids have become recognized for their role in signal transduction and cell regulation. Investigation of sphingolipid-mediated biology has revealed ceramide as an important regulator of cellular processes including terminal differentiation, cell cycle arrest, cellular senescence, and cell death. The action of several extracellular agents and insults such as tumor necrosis factor α, Fas ligands, and chemotherapeutic agents results in the activation of an intracellular sphingomylinase which acts on membrane sphingomyelin and generates ceramide. Mechanisms for ceramide action involve regulation of protein phosphorylation via activation of a protein phosphatase and a protein kinase. Ceramide has also been shown to regulate multiple downstream targets such as activation of the proteases involved in apoptosis, stress-activated kinases, and the retinoblastoma gene product which causes cell cycle arrest. These effects appear to result in profound changes in cell growth behavior and support a role for ceramide as a pluripotent mediator of intracellular stress responses. KEY WORDS: sphingomyelin, ceramide, CAPP, CAPK, PKCζ, NF-κB, SAPK.

INTRODUCTION In the past, lipids were viewed largely as structural building blocks of the membrane. They are now recognized as precursors of bioactive molecules that are generated in cells following

† Corresponding Author: Department of Biochemistry; Medical University of South Carolina, 171 Ashley Ave. Charleston, SC 29425. Tel.: 843 792 4321. Fax: 843 792 4322.

88 S.D.LONG AND Y.A.HANNUN

stimulation of cell-surface receptors and function as second messengers and bioeffector substrates. Sphingolipids, a diverse class of biomolecules found in all eukaryotic membranes, have recently been recognized as important mediators of membrane signal transduction pathways (Hannun et al., 1986; Hannun and Bell, 1989; Hannun, 1994). They have been shown to play a role in such crucial cellular functions as regulation of cell growth, differentiation, oncogenesis, and apoptosis (Kolesnick, 1991; Dobrowsky and Hannun, 1994; Hannun and Linardic, 1994). Since the discovery that sphingosine and lysosphingolipids are potent inhibitors of protein kinase C (PKC), products of sphingolipid hydrolysis have been demonstrated to serve as “lipid second messengers” in a number of pathways (Hannun and Bell, 1989; Kolesnick, 1991; Dbaio et al., 1993; Merrill et al., 1993; Chen et al., 1995). Evaluation of sphingolipid turnover as a mechanism for cell regulation led to the discovery of the sphingomyelin cycle in which activation of a neutral sphingomyelinase leads to the breakdown of sphingomyelin and the generation of phosphocholine and ceramide (Okazaki et al., 1990; Kim et al., 1991). The cycle is completed with the resynthesis of sphingomyelin, presumably by the transfer of a choline phosphate headgroup to ceramide. As such, the sphingomyelin cycle emerges as a sphingolipid analog of the phosphatidylinositol cycle. Various cytokines, hormones, and growth factors are known to induce the hydrolysis of membrane sphingomyelin following ligand binding of receptors. Ceramide, a potent second messenger generated by activation of this pathway, has been identified as an important mediator of growth inhibition, c-myc down-regulation, apoptosis, and the activation of nuclear factor κB (Kolesnick and Golde, 1994; Liscovitch and Cantley, 1994). Despite the numerous reports demonstrating biologic activity for ceramide in vivo, there is no clearly defined intracellular target for ceramide activity. In addition to ceramide, the immediate product of sphingomyelin hydrolysis, attention has also focused on sphingosine, the backbone sphingoid base of all sphingolipids and on sphingosine 1-phosphate. Sphingosine has been shown to modulate numerous cellular functions, including inhibition of protein kinase C activity. Multiple biochemical targets and biological activities of sphingosine have been identified through the use of sphingosine as a pharmacologic agent. In addition to playing a role in growth suppression in PKC dependent and independent pathways, sphingosine has been demonstrated to regulate diacylglycerol and phosphatidic acid levels, calcium release, receptor tyrosine kinase activities, casein kinase II, and to modulate the activity of endogenous protein kinases (Hannun et al., 1986; Hannun and Bell, 1989). On the other hand sphingosine-1-phosphate, another product of sphingosine metabolism, has been shown to induce mitogenesis and to inhibit cell motility and phagokinesis of tumor cells (Zhang et al., 1991; Sadahira et al., 1992). Thus, a biologic role for sphingolipids and their metabolites, including ceramide, sphingosine-1-phosphate, and sphingosine, in cell regulation is becoming increasingly apparent. It is likely that sphingolipids are components of multiple signalling cascades, generating a variety of lipid second messengers which are active participants in cellular metabolism. However, determining the biological functions and mode of action of individual sphingolipids remains difficult to elucidate because of the great variety of sphingolipids present in multicellular eukaryotes. There is increasing evidence supporting the existence of novel lipid signal transduction pathways that may be specifically involved in stress induced apoptosis. Ceramide has been proposed to play a role in the initiation of apoptosis and other stress responses in mammalian

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Table 2.1 Systems involving sphingolipid signalling in the stress response.

cells. Sphingolipids are also proving to be critcal in the yeast stress responses. In the following chapter, we summarize the current understanding of the role sphingolipids play in stress responses and apoptosis. SPHINGOLIPIDS ARE ESSENTIAL FOR CELL VIABILITY AND STRESS RESPONSES IN YEAST AND IN MAMMALIAN CELLS Studies in yeast provided the first genetic evidence in any species that sphingolipids are necessary for survival. In contrast to the sphingolipid diversity in mammalian cells, the sphingolipids of Saccharomyces cerevisiae are limited to one major and two minor types of structurally related sphingolipids (Smith and Lester, 1974). The simplicity of this unicellular eukaryote serves as a model system in which to study sphingolipid biology and function. In 1983, Lester and colleagues isolated a sphingolipid deficient mutant in a strain of S.cerevisiae which requires a sphingolipid long chain base, such as phytosphingosine, for growth and viability (Wells and Lester, 1983). The defect in these long chain base deficient (lcb) strains was ultimately shown to be a defect in the gene coding for serine palmitoyltransferase, the first enzyme in the sphingolipid synthetic pathway. Suppressor strains were selected which bypass the requirement for exogenous long chain bases because of a mutation in a suppressor gene termed SLC1 (sphingolipid compensation) (Dickson et al., 1990). These SLC strains make a set of novel glycerolipids which contain the same polar head groups found in yeast sphingolipids; thus demonstrating that the viability function of sphingolipids in yeast resides in the head group (Lester et al., 1993). These novel phospholipids structurally mimic sphingolipids and thereby may allow cell growth. The strain is, however, capable of making the normal species and levels of sphingolipids if phytosphingosine is added to the culture medium. Hence the SLC strains are of great value in understanding sphingolipid function. In addition, the study of SLC strains has revealed that sphingolipids are required for the cells to respond to certain environmental stresses (Patton et al., 1992). Strains lacking sphingolipids (phytosphingosine omitted from the culture medium) were unable to grow


under extremes of pH, temperature, and osmotic stress. Indeed, the product of an SLC suppressor gene permitted growth without sphingolipids, yet only in a limited range of environments. Outside this range, sphingolipids appeared to be an essential component of the cell’s response and adaptive mechanisms which allow for growth under stressful conditions. Sphingolipids have also been shown to be essential for growth in mammalian cells. Hanada et al. (1990) isolated a temperature-sensitive Chinese hamster ovary cell mutant (strain SPB-1) with a thermolabile serine palmitoyltransferase. When the mutant strain was cultured at non-permissive temperatures, de novo sphingolipid synthesis ceased and the growth rate gradually decreased suggesting that the deficiency in sphingolipids was responsible for the temperature sensitive growth of the mutant cells. Exogenous sphingosine restored the sphingomyelin and ganglioside sialyl lactosylceramide (GM3) contents to normal levels and allowed the mutant cells to grow even at the non-permissive temperature (Hanada et al., 1992). Similarly, exogenous sphingomyelin suppressed temperature sensitivity of the SB-1 growth and restored the sphingomyelin content 100% while GM3 content was restored to only 50% of the parental levels. In contrast, the addition of glucosylceramide, which restored GM3 but not sphingomyelin levels failed to suppress the temperature sensitivity. Thus, the results indicated that the lack of sphingomyelin is primarily responsible for the temperature sensitivity of the mutant cell growth demonstrating the essential role of sphingolipids in growth and viability. The vital importance of sphingomyelin in mammalian cells and inositol phosphorylceramides in yeast cells has become increasingly apparent and may play similar roles in generating ceramide as a potent regulator of signal transduction. CERAMIDE, THE CARDINAL LIPID OF SPHINGOLIPID METABOLISM The discovery of the sphingomyelin cycle led to the identification of ceramide as the primary metabolite of agonist induced sphingomyelin hydrolysis. Ceramide has been studied extensively and its role as a bioeffector is becoming increasingly apparent. Generated via the sphingomyelin pathway, ceramide emerges as a major mediator of growth suppression through induction of differentiation, initiation of apoptosis, activation of tumor suppressors, and induction of specific cell cycle arrest. These activities have been studied in a myriad of cell lines in which agonist induced ceramide production can be correlated with the effect of exogenous ceramides on cell growth and differentiation. Growth suppression was one of the earliest biologic effects attributed to ceramide. This was first demonstrated in human leukemia HL60 cells where treatment with 1α, 25-dihydroxyvitamin D3 stimulated sphingomyelin (SM) hydrolysis within 2 to 4 hours generating choline-phosphate and ceramide (Okazaki et al., 1989). Since these initial studies with HL60 cells, the list of inducers known to act via the SM cycle continues to expand and thus, a novel cell regulation pathway emerges with the lipid second messenger, ceramide, at the center.


BIOLOGIC ACTIVITY OF CERAMIDE AS A REGULATOR OF THE APOPTOTIC RESPONSE AND GROWTH SUPPRESSION Ceramide and apoptosis The induction of programmed cell death involves the activation of biochemical mechanisms leading to regulated cell death or apoptosis. Various studies have identified ceramide as a possible mediator of apoptosis in response to the action of multiple agents and insults including γ-interferon, the presence of hypoxia, Fas ligand,and tumor necrosisfactor-α (TNFα) (Kim et al., 1991; Dressler et al., 1992; Wiegmann et al., 1992; Yanga and Watson, 1992; Tepper et al., 1994). Mammalian cells have an intrinsic biochemical response that allows them to repair the damage caused by these agents by undergoing cell cycle arrest or, if the damage is irreversible, apoptosis is initiated. TNFα has been shown to induce activation of neutral sphingomyelinase through the activation of phospholipase A2 and the generation of arachidonic acid as an intermediate coupling mechanism (Jayadev et al., 1994). As a result of sphingomyelin hydrolysis, the elevation of intracellular ceramide levels is thought to mediate the effects of TNFα-induced apoptosis. Studies carried out by Obeid and coworkers demonstrated that in U937 monoblastic leukemia cells, TNFα-induced SM hydrolysis and ceramide generation preceded the cellular effects of TNFα-induced growth suppression and DNA fragmentation (Obeid et al., 1993). In addition, exogenous addition of ceramide was found to mimic the apoptotic effects of TNFα in a time and dose dependent manner. The structurally related lipid, dihyrdroceramide, did not induce apoptosis, suggesting the action of ceramide is specific. However, the question remained whether ceramide itself was eliciting the stress responses, or if subsequent degradation to other sphingolipids such as sphingosine or sphingosine-1-phosphate was occurring. Although ceramide inhibits phospholipase D activation and sphingosine induces its activation (Venable et al., 1994), in many cell systems ceramide and sphingosine induce similar activities including growth suppression and induction of Rb dephosphorylation (Dbaio et al., 1995). Recent studies using compounds that result in accumulation of endogenous ceramide such as bacterial sphingomyelinase, PDMP (a cerebroside synthase inhibitor), and D-MAPP (which inhibits ceramidase) distinguished the effects of endogenous ceramide from those of sphingosine (Inokuchi et al., 1989; Bielawska et al., 1992a, b). The results demonstrated that an increase in ceramide alone without further metabolism could elicit the same cellular activities as exogenously added ceramide analogs. Therefore, a direct role for ceramide in mediating apoptosis and growth suppression is supported by indirect metabolic manipulation of endogenous amounts of ceramide. Ceramide and cell cycle arrest Among the numerous biological effects elicited by ceramide, cell cycle arrest has been recently discovered as an integral component. The progression of cell cycle is tightly regulated by multiple genes and gene products in response to proliferation and antiproliferation. Since cultured cells depend on various serum growth factors for continued growth, serum deprivation becomes a powerful mechanism by which cells can be arrested in the cell cycle and induced to undergo programmed cell death. In a study carried out by


Jayadev et al. (1995) ceramide was found to induce a significant block in cell cycle progression in G0/G1 accompanied by apoptosis. Serum starvation of Molt-4 leukemia cells led to a remarkable increase in endogenous ceramide levels which paralleled apoptosis and cell cycle arrest. However, investigation of the mechanism of increased ceramide production led to the finding that serum withdrawal appears to stimulate a signalling cascade distinct from the TNFα stimulation of the sphingomyelin cycle. Serum deprivation resulted in substantial activation of a particulate, magnesium-dependent, neutral sphingomyelinase rather than the magnesium-independent enzyme observed with TNFα. These studies identified a novel role for ceramide in cell cycle arrest and raised the possibility that ceramide modulates the endogenous machinery regulating cell cycle progression. The product of the retinoblastoma gene, a tumor suppressor protein known as Rb, has emerged as a possible target for serum deprivation-induced cell cycle arrest. Recent studies have also demonstrated that Rb may be an important target of ceramide regulation. Hence mechanistically, ceramide-induced cell cycle arrest could be mediated by the Rb protein since phosphorylation of Rb has been found to be regulated by ceramide. Molt-4 cells treated with C6-ceramide showed significant Rb dephosphorylation in a time frame which precedes G0/G1 arrest and at ceramide concentrations comparable to endogenous levels which produce arrest (Dbaio et al., 1995). The specificity of this effect was confirmed by the use of several other lipids, including dihydro-C6-ceramide which failed to change the phosphorylation status of Rb. Furthermore, experiments performed in different cell lines either containing or lacking functional Rb demonstrated that the presence of functional and active Rb protein is essential for mediating the effects of ceramide on growth suppression and cell cycle arrest. Taken together, these studies show that Rb is a downstream target of ceramide and that ceramide-induced Rb dephosphorylation is a biochemical precursor to cell cycle arrest. Ceramide and senescence Our knowledge of lipid mediated signal transduction pathways in cellular senescence is modest. Cellular senescence is defined as the limited capacity of cells to undergo population doublings due to the inability to respond to mitogenic signals with DNA synthesis, growth, and proliferation (Goldstein, 1990; Kirkland, 1992). The mechanism by which senescent cells fail to respond to mitogenic stimuli remains, however, poorly understood. Preliminary studies have begun to indicate changes in components of the sphingomyelin cycle with respect to cellular aging. Studies by Venable and colleagues provided direct evidence that defects in the DAG/PKC pathway underlies the mitogenic defect in cell senescence (Venable et al., 1994). Unlike their young counterparts, senescent human diploid fibroblasts (HDF) did not respond to serum-induced activation of phospholipase D (PLD) which resulted in an inability to generate a sustained diacylglycerol signal. Without DAG production, PKC was not translocated resulting in failure to transcribe c-fos and activate AP-1, a transcription factor required for cell replication. They also investigated the possible connection between the ceramide pathway and PLD/ DAG pathway and found that the inability of senescent cells to activate PLD may be attributed to elevated levels of ceramide. During senescence, endogenous levels of ceramide increased considerably (4-fold) and specifically (compared to other lipids) (Venable et al., 1995). The elevated ceramide in


senescent cells was a result of increased neutral sphingomyelinase activity (8–10 fold). The observed increases in SMase activity and ceramide appeared to be stable and prolonged as opposed to the transient signalling increases in response to inducers of apoptosis and differentiation. Moreover, the addition of C6-ceramide to young cells induced a senescent phenotype characterized by the inability to undergo DNA synthesis and mitogenesis. In addition, exogenous ceramide at concentrations that mimic endogenous levels in senescence also induced Rb dephosphorylation and inhibited seruminduced AP-1 activation in young HDF, recapitulating the established biochemical and molecular changes of cell senescence. Based on these studies, ceramide appears to be one of the first biologically active molecules regulating a biochemical event that distinguishes senescence from cell cycle growth arrest. Important insight into the molecular mechanisms involved in cellular senescence will come with further understanding of the sphingomyelinase/ceramide pathway and its mechanism of action on cellular regulation. Ceramide and differentiation The role of the sphingomyelin cycle in the regulation of differentiation and cell growth was first described in HL60 leukemia cells (Okazaki et al., 1989; 1990; Kim et al., 1991). Treatment with the cell permeable ceramide analog, C2-ceramide, was sufficient to induce monocytic differentiation and inhibit growth of these cells, mimicking the action of TNFα, 1α, 25-dihydroxyvitamin D3, and γ-interferon in inducing monocytic differentiation. The effects of C2-ceramide were probably not due to metabolic conversion to other sphingolipids since neither sphingosine or N-ethylsphingosine induced differentiation in HL60 cells. Ceramide-induced cell differentiation has been extended to other cell systems as well. In T9 glioma cells, nerve growth factor (NGF), which promotes growth inhibition and differentiation, has been shown to activate the sphingomyelin cycle resulting in ceramide generation (Dobrowsky et al., 1994). The addition of C2-ceramide mimicked the effects of NGF and resulted in a dose-dependent inhibition of proliferation and induction of differentiation. The protooncogene, c-myc, which plays a central role in cell proliferation and differendation, has been identified as a candidate downstream effector of ceramideinduced differentiation (Kim et al., 1991; Wolff et al., 1994). In the HL60 cell line, treament with C2-ceramide, as well as TNFα, resulted in a rapid downregulation of c-myc mRNA. The mechanism by which ceramide exerted its effect was demonstrated to be a block in transcriptional elongation similar to the effect produced by TNFα. Recent work has suggested that ceramide’s effects are mediated via ceramide activated protein phosphatase (CAPP) (Wolff et al., 1994). A role for CAPP in the ceramide signalling pathway was supported by the observation that both CAPP activation and c-myc downregulation exhibit similar specificities for various ceramide analogs and stereoisomers. In addition, the effects of ceramide were abolished when cells were treated with okadaic acid, an inhibitor of CAPP. These studies provide significant insight for the mechanism by which ceramide regulates cell proliferation and differentiation and show that CAPP may function as an intracellular target for the action of ceramide.


A POTENTIAL ROLE FOR SPHINGOLIPIDS IN INFLAMMATION AND THE IMMUNE RESPONSE Ceramide and the immune response TNFα is now known to be one of the most pleiotropic cytokines and is viewed as a principal mediator of a large number of cellular responses including the immuno-inflammatory response. Most of the biological effects of TNFα are triggered by activation of a variety of genes in a multitude of target cells. Induction of many genes by TNFα is mediated at least in part, by the activation of a family of transcription factors known collectively as NF-κB (Lowenthal et al., 1989; Lenardo and Baltimore, 1989; Osborn et al., 1989; Molitor et al., 1990); a cellular response that links cell surface receptor activation to transcriptional events in the nucleus. TNFα is one of the few cytokines known to stimulate the translocation of NFκB from the cytosol to the nucleus where the transcription factor exerts either a positive or negative control over cellular genes. Activation of NF-κB involves the release of the inhibitory subunit Iκ-B from a cytoplasmic complex containing the DNA binding subunit p50 and the regulatory subunit Rel A. The Iκ-B proteins inhibit DNA binding and prevent nuclear uptake of NF-κB complexes (Baeuerle and Baltimore, 1994). The role of the sphingomyelin/ceramide pathway in the activation of NF-κB remains uncertain. Exogenous ceramides have for the most part been shown not to induce nuclear translocation and activation of NF-κB (Dbaibo et al., 1993). However, reports do exist of sphingomyelinases activating NF-κB in a similar manner as TNFα, suggesting a role for ceramide (Schutz et al., 1992; Lozano et al., 1994). Addition of acidic or neutral sphingomyelinases to cell homogenates at pH 5.0 and 7.4, respectively, activated NF-κB to an extent similar to that produced by TNFα. Further investigation revealed that, although both diacylglycerol and ceramide were produced in response to TNFα, the mechanism of hydrolysis of sphingomyelin was thought to occur via TNFα activation of PC-PLC and subsequent generation of DAG. The acid sphingomyelinase appeared to be the target since sphingomyelin hydrolysis in response to DAG occurred only at pH 5.0 in vitro. More recent studies have demonstrated in vivo that inhibition of acid sphingomyelinase by SR33557, did not block the TNFα induced activation of NF-κB. (Higuchi et al., 1996). In addition, in Niemann-Pick fibroblast cells which lack acid SMase, NF-κB was still activated in response to TNFα (Kuno et al., 1994). Based on studies from Moscat and coworkers, a model has been proposed where ceramide is the second messenger that activates NF-κB by stimulation of PKCζ, which has been shown to phosphorylate and inactivate Iκ-B (Lozano et al., 1994). They demonstrated that addition of exogenous sphingomyelinase to NIH3T3 fibroblasts transactivated a κBdependent chloramphenicol acetyltranserfase reporter plasmid, in a similar manner as TNFα, and PC-PLC. Furthermore, κB transactivation by TNFα, PC-PLC, and sphingomyelinase was dramatically inhibited by transfection of a PKCζ dominant negative mutant, suggesting a role for PKCζ in activation of NF-κB. PKCζ was also shown to be activated by ceramide in vitro and in vivo by treatment with sphingomyelinase. Taken together, these results suggest that PKCζ is a component of the SM signalling pathway leading to NF-κB activation. Although, ceramide may indeed directly activate PKCζ in vivo, whether this leads to induction of NF-κB translocation by ceramide remains to be seen.


An important point raised by these studies concerns the role of ceramide in modulating NFκB activation. Although the importance of ceramide in activation of NF-κB is far from clear, it is conceivable that TNFα launches a multistep process in which numerous signals, including ceramide, are necessary but none are sufficient for activation of NF-κB. Ceramide and inflammation Studies in human fibroblasts showed that ceramide and sphingosine markedly enhanced the production of PGE2 in response to IL-1 stimulation (Ballou et al., 1992). Sphingomyelin hydrolysis and ceramide generation did, indeed, occur in response to IL-1 treatment. Further investigation of the mechanism of ceramide’s effect on PGE2 synthesis revealed that ceramide or sphingosine treatment resulted in a considerable increase in the expression of cyclooxygenase, the rate limiting enzyme in the synthesis of PGE2. Interestingly, ceramide or sphingosine alone were unable to elicit PGE2 production, implying the possibility that molecules other than ceramide may be the physiological mediators of the inflammatory response. CERAMIDE: A KEY COMPONENT OF INTRACELLULAR STRESS RESPONSE PATHWAYS A new subfamily of protein kinases has recently been identified as stress-activated protein kinases (SAPKs) also known as Jun nuclear kinases or JNKs (Kyriakis et al., 1994; Sluss et al., 1994; Westwick et al., 1994). They are related to the mitogen activated protein (MAP) kinases, but activated preferentially by cellular stress and tumor necrosis factor-α (TNF). The TNFα signalling pathway which activates the SAPKs leads to the stimulation of a specific set of transcription factors. SAPK phosphorylates and activates c-Jun and ATF-2 which in turn preferentially induce genes with specific non-consensus AP-1 binding sites, such as the c-jun gene itself (Hibi et al., 1993; Derijard et al., 1994; Gupta et al., 1995). TNFαstimulated sphingomyelin hydrolysis and generation of ceramide has been suggested as an early step in the SAPK-activation pathway leading to apoptosis. Addition of exogenous ceramides or sphingomyelinases induced SAPK activity and enhanced expression of the c-jun gene suggesting that ceramide functions upstream of SAPK. Furthermore, a dominant negative mutant of c-jun blocked the stress-induced apoptosis but did not inhibit the generation of ceramide or SAPK activation suggesting a role for SAPK in mediating apoptosis possibly downstream of ceramide. However, another study dissociates SAPK activation from induction of apoptosis (Liu et al., 1996). Therefore, whether SAPK is indeed involved in apoptosis remains to be resolved. Also, the role of ceramide in this process remains to be defined. TNFα has been shown to have a dual function in mammalian cells, inducing both inflammation and apoptosis. Ceramide has been suggested to regulate both responses. However, the inflammatory response is thought to be induced via the MAPK cascade (Yao et al., 1995) whereas the apoptotic or stress response is proposed to be activated through the SAPK pathway (Westwick et al., 1995; Verheij et al., 1996). Recently KSR (kinase suppressor of ras) was identified as CAPK (ceramide activated protein kinase) (Zhang et al.,


Figure 2.1 Ceramide-mediated stress responses. Ceramide has been shown to regulate differentiation, cell cycle arrest, and apoptotic cell death. The specific biologic response appears to be determined by the particular cell type. Mechanisms for ceramide action involve activation of a cytosolic serine/threonine protein phosphatase 2A (CAPP), a membrane bound serine/threonine protein kinase (CAPK/KSR), and PKC ζ Downstream targets of ceramide include the protooncogene, c-myc, the retinoblastome gene product (Rb), and the death proteases (prICE/ YAMA/CPP32). The dotted lines represent proposed pathways which are not clearly defined.

1997) where ceramide stimulates KSR/CAP kinase to complex with and phosphorylate Raf-1, initiating signalling down the MAPK pathway. The essential signalling events linking Fas receptor to apoptosis have been the subject of intense investigation. The structural and physiological similarities between the TNFR and Fas led to the possibility that ceramide also participates in apoptosis triggered by the Fas antigen. In a recent report by Tepper and coworkers, crosslinking of the Fas antigen with agonist resulted in a coordinate increase in sphingomyelinase activity and ceramide supporting a role for ceramide in Fas induced apoptosis (Tepper et al., 1995). Further delineation of the Fas signalling cascade revealed Fas activation of the SAPKs, p38 and JNK, but not activation of the mitogen activated kinases (Juo et al., 1997). Fas-mediated apoptosis also requires the action of interleukin-lβ converting enzyme (ICE) or ICE/CED-3 family proteases, based on the ability of the cowpox viral protein CrmA, which inhibits ICE proteases, and specific tetrapeptide ICE inhibitors to inhibit Fas-induced death (Enari et al., 1995; 1996; Los et al., 1995; Tewari and Dixit, 1995). In addi tion, the inhibitors also blocked Fas-induced activation of p38, demonstrating that Fas-dependent activation of p38 requires ICE/CED-3 family members and conversely that the proteases act as upstream regulators of p38 during Fas signalling. Interestingly, ceramide has been shown to activate the same family of ICE proteases specifically prICE/YAMA/CPP32, the protease responsible for cleavage of poly (adenosine diphosphate-ribose) polymerase (PARP) (Smyth et al., 1996). Overexpression of the cell death regulator Bcl-2 resulted in inhibition of ceramide activation of prICE and induction of apoptosis (Dbaibo et al., 1997) suggesting that Bcl-2 functions downstream of ceramide. Moreover, overexpression of Bcl-2 did not attenuate the increase in ceramide levels with the addition of extracellular agents. The mechanism by


which ceramide directly or indirectly activates the protease remains to be determined, although potential candidates for direct targets of ceramide action have begun to emerge. MECHANISMS OF CERAMIDE ACTION The sphingolipid signalling pathway is thought to mediate downstream biologic effects via modulation of cellular phosphoprotein metabolism. Ceramide has been demonstrated to activate both a membrane bound kinase activity as well as a cytosolic protein phosphatase activity. More recently the protein kinase C ζ has been suggested to play a critical role in the SMase signalling pathway leading to κB-dependent promoter activation. This observation has led to the investigation of ceramide as the second messenger that activates NF-κB by stimulation of PKCζ. Ceramide activated protein phosphatase The search for a direct molecular target of ceramide is ongoing. Biologically relevant candidates should be activated by ceramide in vitro in addition to mediating the most proximal effects of ceramide in vivo. The identification of a serine-threonine protein phosphatase in rat T9 glioma cells led to the most promising suitor for ceramide action (Dobrowsky et al., 1993). Ceramide activated protein phosphatase (CAPP) has been shown to be activated by both native (C18) and cell permeable (C2) ceramides but not by sphingosine or sphingomyelin in vitro (Dobrowsky and Hannun, 1992). CAPP, related to the family of PP2A phosphatases, has no cation requirement and is potently inhibited by okadaic acid. Studies in S. cerevisiae where ceramide-induced growth inhibition has been established, the existence of a ceramide-dependent serine/threonine phosphatase has also been proven in yeast (Fishbein et al., 1993). The biochemical characteristics of the yeast CAPP are similar to that of mammalian CAPP indicating ceramide-mediated cell regulation is conserved in all eukaryotes and hence, of fundamental importance. Recent studies have begun to implicate CAPP as a potential mediator of certain cellular activities of ceramide, including downregulation of the proto-oncogene c-myc and apoptosis. Primarily, the specificity of activation of CAPP in vitro closely matches the specificity of ceramide-induced c-myc gene regulation and apoptosis. CAPP is activated in vitro by ceramide but not by dihydroceramide which is inactive in eliciting ceramide activities in cells (Dobrowsky and Hannun, 1992). In addition, low concentrations of okadaic acid can inhibit the regulatory effect of ceramide on c-myc and apoptosis, providing further evidence of a role for PP2A-type phosphatase in ceramide signalling. Both ceramide and TNFα induce c-myc downregulation through a similar mechanism involving a block to transcription (Wolff et al., 1994) which appears to involve an okadaic acid-inhibited phosphatase. Although, relevant physiological substrates for CAPP have yet to be identified, the possibility of CAPP playing a role in TNFα signal transduction via activation by ceramide is emanate.


Ceramide-activated protein kinase Ceramide activated protein kinase (CAPK) is an exclusively membrane bound enzyme that is a member of an emerging family of proline directed serine/threonine protein kinases (Mathias et al., 1991; Joseph et al., 1993). The 97-kDa autophosphorylating kinase is activated by C8-ceramide in a cell free system suggesting the possibility that CAPK may serve as a direct target for the action of ceramide. However, upon renaturation from SDS gels, the kinase is no longer ceramide responsive (Liu et al., 1994). In a recent study by Zhang et al. (1997) the kinase suppressor of Ras (KSR) was identified genetically in C. elegans and Drosophilia, as CAPK. They demonstrate that recombinant KSR displays all of the previously described properties of CAPK. The designation of KSR as CAPK may allow for more precise molecular ordering of transmembrane signalling events. However, the specificity of ceramide activation of CAPK and its role in ceramide signalling awaits further investigation. Protein kinase C ζ Yet a third target for ceramide action has been proposed -PKC ζ The ζ isoform of PKC is distinguished from other isoforms (except λ) with regard to its lack of activation by DAG or phorbol esters. Even more notable, is the observation by Lozano et al. of direct PKCζ activation by ceramide in vitro (1994). As discussed previously, they suggest that this activation may be important in mediating the regulation of NF-κB in response to TNFα. The kinase may be an immediate target for the lipid second messenger but its importance in regulating the various biological effects of ceramide remains to be elucidated. Thus, there appear to be several candidates that may serve as direct targets for the action of ceramide. Indeed, multiple in vitro targets for ceramide may exist and may mediate distinct biologic activities of ceramide. However, determining the role of these enzymes in mediating the specific cellular activities of ceramide, emerges as the key goal for the elucidation of signal transduction pathways activated by ceramide. LESSONS FROM YEAST In mammalian systems sphingolipids play a part in important cellular functions and in yeast they are known to be essential for growth and viability. Studies in mutant strains of S. cerevisiae have identified sphingolipids as essential components of growth in high osmolarity conditions suggesting that the lipids play an important role in the signalling cascade regulating the osmostress response. The HOG1 kinase signalling cascade found in yeast is similar to the stress-activated kinase pathway described in mammalian cells. When yeast cells are confronted with high osmolarity, they induce the synthesis of glycerol to increase their internal osmolarity in an effort to re-establish osmotic equilibrium. The signalling pathway that mediates this response uses the PBS2 and HOG1 genes which code for a MEK and a MAPK homolog respectively and are required for cell growth in high osmolarity medium. By comparison, one could predict that sphingolipids activate the yeast HOG1 kinase in a similar manner to ceramide activation of SAPK in mammalian cells. Furthermore the p38 MAPK presents substantial homology to HOG1 (Kyriakis et al., 1994) and like


HOG1, can be activated by changes in osmolarity but the mechanism by which activation occurs remains to be determined. Studies on the yeast MAPK stress responses have the potential to identify the function of sphingolipids in regulating the osmostress response which can then be easily applied to mammalian stress activated pathways. CONCLUSIONS AND FUTURE DIRECTIONS All living cells elicit coordinated biochemical responses that will either enhance cell survival or lead to cell death when cells are exposed to adverse environmental conditions. This ubiquitous reaction to metabolic disturbances is designated the stress response. The intracellular stress signals generated provide the necessary network which allow the occurrence of both specific and general responses to stress challenges. The decision for cellular death or life may depend on the integration of multiple signals generated from many signal transduction pathways. Membrane lipids functioning as ‘lipid second messengers’ have emerged as key components in such fundamental mechanisms regulating the stress responses, growth suppression, and programmed cell death. The endeavors of many years of research are beginning to define a novel sphingolipid-mediated signal transduction pathway where several of these stress-activated biochemical pathways converge on ceramide. Although the basic blueprint of this novel signalling pathway can be outlined to illustrate sphingomyelin hydrolysis and the formation of ceramide which directly activates a number of cellular targets, the ultimate biological response depends on the context in which the signal, ceramide, is generated. Through the definition of CAPP, CAPK, and PKZζ as intracellular targets for ceramide action, we have begun to understand the mechanism by which the proximal effects of ceramide can be mediated such as the regulation of c-myc expression, Rb phosphorylation, cyclooxygenase expression, and NF-κB translocation. However, we have yet to define direct endogenous substrates of these proteins. The yeast model provides an ideal system in which to dissect the components of signal transduction pathways through the selection of mutants and cloning of involved genes. Studies in S.cerevisiae have demonstrated that ceramide-mediated growth regulation is conserved in all eukaryotes, thus proving its fundamental importance. Thus, yeast molecular genetics will provide valuable information in identifying proximal targets for the action of ceramide and determining their role in ceramide signalling. Activation of the SAPKs by ceramide and sphingomyelinase provide strong evidence that ceramide, or a related metabolite, functions as the lipid second messenger in TNF signalling resulting in the activation of stress kinases. However, the mechanism that connects the upstream messengers to the activation of the cytoplasmic and nuclear kinases remains elusive. Evidence does exist suggesting that the ICE family of proteases may be involved in lipid activation of stress kinase pathways. First, ICE and CPP32 have been shown to activate JNK and p38 stress kinases leading to apoptosis. Second, ceramide has been shown to induce activation of prICE, the protease that cleaves PARP and the activation is inhibited by Bcl2. These studies support a distinct role for proteases in the mammalian stress activated pathway, placing them downstream of ceramide and upstream of the SAPKs.


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3. RADIATION RESPONSE PATHWAYS AND APOPTOSIS MARTIN F.LAVIN*† *The Queensland Cancer Fund Research Unit, The Queensland Institute of Medical Research, Department of Surgery, University of Queensland, PO Box Royal Brisbane Hospital, Herston, Brisbane 4029, Australia

KEY WORDS: radiation, DNA damage, nuclear and membrane signalling, cell cycle.

INTRODUCTION Engagement of the Fas/APO-1 and TNF receptors with their respective ligands FasL and TNF leads to the initiation of apoptosis by a number of transducing molecules some of which have been identified (Stanger et al., 1995; Hsu et al., 1995). After ligand binding or crosslinking of the receptor, activation is initiated by association of FADD/MORT1 (Fasassociated protein with death domain) with a homologous region on the Fas receptor called the death domain (Boldin et al., 1996; Chinnaiyan et al., 1995). In the case of the TNF receptor, association occurs with TRADD (TNF-RI-associated death domain protein) (Boldin et al., 1996), but FADD also associates with this complex at least in some cell types under conditions that lead to apoptosis. Another protein RIP also associates with both receptors and may be processed by one of several other receptor associated proteins (Kischkel et al., 1995). After these initial changes in the death signalling complex, a cysteine protease MACH/FLICE (caspase 8), is activated and since it is capable of activating a series of other caspases by cleavage it has been suggested that it is at the peak of a cascade of activating reactions (Alnemri et al., 1996). The end result is a series of active caspases which are thought to target specific substrates in the cell (Martin and Green, 1996). Of these caspase 3 has been shown to cleave poly (ADP-ribose) polymerase (PARP) (Lazebnik et al., 1994; Fernandez-Alnemri et al., 1995b) the catalytic subunit of DNA-dependent protein kinase (DNA-PKcs) (Song et al., 1996) U1–70kDa (Casciola-Rosen et al., 1994) and PKCδ (Emoto et al., 1995). These and several other caspase substrates appear to be key targets in the nucleus, nuclear scaffold, cytoplasm and cytoskeleton during the onset of apoptosis (Nicholson, 1997). In summary, in the case of anti-Fas and TNFmediated apoptosis, a complex series of signalling steps are initiated from the receptor which leads to protease activation, substrate degradation and ultimately apoptosis.

† Corresponding Author: Tel: 617 3362 0341. Fax: 617 3362 0106. e-mail: [email protected]

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It seems likely that apoptosis induced by a variety of agents such as ionizing radiation, etoposide and glucocorticoids also involves some form of receptor activation at least as part of the process. The cell radiation response is more complex than simply responding to DNA damage by repairing lesions. It is now evident that a number of signalling pathways can be activated in response to radiation damage involving both the activation or inactivation of existing proteins as well as the induction of a number of genes (Knebel et al., 1996; Fornace, 1992). Some of these pathways are activated in response to DNA damage, but signalling is not exclusively initiated by such lesions since it has been demonstrated that membrane receptors can mediate the transmission of such signals (Karin and Hunter, 1995). Signals initiated in these pathways can elicit DNA repair, lead to cell cycle arrest or direct cells to undergo apoptosis. The decision to induce cell cycle arrest or apoptosis is dependent upon the expression of other gene products that control cell growth and proliferation (Clarke et al., 1993; Linke et al., 1997). In this chapter radiation signal transduction and its role in apoptosis will be discussed. RADIATION-INDUCED DNA DAMAGE DNA appears to be the major target for radiation-induced cell killing (Ward 1985). The lesions induced by ionizing radiation include single and double strand breaks in the phosphodiester backbone of DNA, base and sugar modification as well as cross-links between DNA strands and between DNA and proteins (Van der Schans et al., 1982; Ward, 1985; Téoule, 1987). The strand breaks in DNA arise either directly by interaction with atoms in the phosphodiester backbone causing the ejection of electrons from these atoms or indirectly through water radiolysis which generates free radical species capable of abstracting hydrogen atoms from DNA, resulting in strand breakage (Blok and Lohman, 1973). Single strand interruptions in the DNA backbone are rapidly and efficiently repaired in mammalian cells (Téoule, 1987). The probability of random ionization events being close to one another on opposite DNA strands increases with increasing radiation dose, resulting in double strand breaks in DNA. While a variety of lesions occur in DNA postirradiation it seems likely that the double strand break is the most significant lesion for cell killing (Radford, 1986a, 1986b; Bryant, 1985). The description of a number of human and rodent cell lines characterized by hypersensitivity to radiation and reduced ability to repair double strand breaks in DNA, post-irradiation, provides further support for the importance of this lesion (Badie et al., 1995; LeesMiller et al., 1995). A cell line, 180 BR, derived from a patient with acute lymphoblastic leukaemia and an adverse reaction to radiotherapy, represents the first known example of human fibroblasts that are both hypersensitive to ionizing radiation and deficient in double strand break repair (Badie et al., 1995). While it seems unlikely that the molecular defect in the human genetic disorder ataxiatelangiectasia (A-T) is directly concerned with repair of breaks in DNA there is evidence that approximately 10% of double strand breaks persist up to 72 h postirradiation (Cornforth and Bedford, 1985; Yoray et al., 1995,1997). It is suggested that an intrinsic abnormality in chromatin structure in A-T cells causes a more proficient translation of DNA damage into chromosomal damage accounting for the higher level of chromosome aberrations in these cells (Pandita and Hittleman, 1992, 1994).

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RADIATION INDUCED CELL KILLING Hypersensitivity to radiation is associated with 11 complementation groups in rodent cell mutants and 4 of these are complemented by human genes involved in double strand break repair (Thompson and Jeggo, 1995; Zdzienicka, 1995). Mutations in other groups include those in the DNA-dependent protein kinase (DNA-PK) multi-protein complex; the Ku subunits (Ku 70 and Ku 86) that bind to the free ends of DNA and the catalytic subunit of DNA-PK, DNA-PKcs, which is recruited to DNA breaks and sites of damage by the Ku heterodimer (Gottlieb and Jackson, 1994; Kirchgessner et al., 1995; Taccioli et al., 1993). The scid mutation which occurs in DNA-PKcs (Blunt et al., 1995; Boubnov and Weaver, 1995; Kirchgessner et al., 1995) is characterized not only by a defect in double strand break repair, and as a consequence hypersensitivity to ionizing radiation (Biedermann et al., 1991), but also by defective V(D)J recombinant and immunodeficiency (Bosma et al., 1983; Hendrickson et al., 1988; Lieber et al., 1988). Another gene predisposing to radiation hypersensitivity is the gene (ATM) mutated in the human genetic disorder ataxiatelangiectasia (A-T) (Savitsky et al., 1995; Lavin and Shiloh, 1997). The defect in A-T appears to be largely due to a failure to respond to or recognize DNA damage and as a consequence a fraction of double-strand breaks remain unrepaired (Cornforth and Bedford, 1985; Foray et al., 1997). Meyn (1995) proposed a damage surveillance hypothesis which bestows on ATM the ability to activate an elaborate system responsible for cell cycle arrest, DNA repair and protection against apoptosis induced by DNA damage. In A-T cells, it is suggested that the threshold for p53-induced apoptosis is lowered and apoptosis accounts for the majority of cell death post-irradiation. IMPORTANCE OF PRE-EXISTING PROTEINS Evidence accumulated from cells in culture, cell-free extracts and enucleated cells demonstrate that the factors required for apoptosis are already present in the cell and no new RNA or protein synthesis is required (Duke et al., 1983; Martin and Cotter, 1991; Nakajima et al., 1995; Raff et al., 1993; Sellins and Cohen, 1991). A greater knowledge of this process has helped to resolve the apparent differences that existed between murine thymocytes on the one hand and a variety of human cell lines on the other hand. Several reports have provided evidence that murine thymocytes are dependent on de novo protein synthesis for apoptosis to occur (Wyllie et al., 1984; Sellins and Cohen, 1991; Cohen et al., 1985; McConkey et al., 1988). In these experiments evidence for a reliance on protein synthesis was provided by the use of inhibitors such as cycloheximide (Wyllie et al., 1984; Cohen et al., 1985; McConkey et al., 1988). On the other hand not only did this compound fail to prevent apoptosis in human cells (Chang et al., 1989; Kelley et al., 1992) but it was shown in some cases to exacerbate the process (Baxter and Lavin, 1992; Baxter et al., 1989; Collins et al., 1991; Martin et al., 1990). Inhibitors have been useful to delineate a variety of metabolic steps but have the disadvantage that they are differentially cytotoxic in different cell types. The evidence that emerges in several systems undergoing apoptosis is that pre-existing proteins are of key importance (Chang et al., 1989; Kaufmann, 1989; Nazareth et al., 1991; Song et al., 1996). In this respect comparison can be made to signal transduction pathways involving receptor-ligand interaction which leads to a cascade of events that are largely controlled by

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phosphorylation and activation of proteins such as the caspase cascade (Grupp et al., 1995; Kapeller and Cantley, 1994). We have shown previously that apoptosis induced by either heat treatment or ionizing radiation exposure of human cell lines is accompanied by dephosphorylation of a limited number of specific proteins (Baxter and Lavin, 1992). In addition inhibitors of phosphatases 1 and 2A, okadaic acid and calyculin A, delayed the process of apoptosis in response to a variety of damaging agents (Baxter and Lavin, 1992; Song et al., 1993). At longer times after treatment, okadaic acid, because of its toxicity, induced apoptosis. Ohoka et al., (1993) have also demonstrated, using okadaic acid, that protein dephosphorylation is an essential step for glucocorticoid-induced apoptosis in murine T-cell hybridomas. Paradoxically okadaic acid is also capable of inducing apoptosis in myeloid cells (Boe et al., 1995; Gjertsen et al., 1994; Kiguchi et al., 1994; Lerga et al., 1995). Ishida et al., (1992) demonstrated the late onset of apoptosis in myeloid leukaemia cells exposed to okadaic acid. It is evident that the window of inhibition for okadaic acid depends very much on the cell type, presumably reflecting the responses of the cell to the specific apoptotic stimulus and the stage at which phosphatase activity is critical. Further evidence for the involvement of pre-existing factors is provided from in vitro experiments employing cell-free extracts incubated with nuclei (Martin et al., 1995), These data show that the apoptotic program is predominantly extranuclear with cytoplasmic extracts from cells undergoing apoptosis being capable of causing condensation of chromatin in isolated nuclei. The use of specific inhibitors suggest that these changes are mediated by caspases. While the activation of these caspases by various stimuli is complex it is now evident that mitochondria play a key role in the activation of these enzymes (Orrenius et al., 1997). The role of mitochondria in apoptosis is discussed at length elsewhere in this book (D Green chapter 10). In the present context the opening of mitochondrial megachannels by permeability transitions release an apoptosis-inducing factor which is apparently responsible for the activation of caspases. PLASMA MEMBRANE INVOLVEMENT IN RADIATION-INDUCED SIGNALLING As outlined above, DNA is a major target for cell killing in response to radiation damage. This is achieved by direct interaction with the DNA or by the creation of free radicals that indirectly damage DNA (Ward, 1985). However, as is obvious, H2O, a target for ionization is not confined to the nucleus. While the biological significance remains uncertain, it has been long shown that radiation and the free radicals arising can alter membrane proteins and lipids (Wallach, 1972). Since the plasma membrane contains in excess of 60% protein, these molecules are clearly potential targets. The oxidation state of disulfide links is altered by radiation expos ure and multiple amino acid residues have been shown to be damaged when proteins are irradiated in the solid state or in solution (Wallach, 1972). Membrane lipids can also be altered by the generation of lipid peroxides. However, radiation doses that alter the properties of membranes such as permeability are considerably higher (100–500 Gy) than those that exert significant damage in DNA (Kankura et al., 1969). More recently with the advance of more sensitive methodology it has been possible to detect other aspects of the response of cells to radiation damage at the level of the membrane and cytoplasm (Devary et al., 1993; Karin and Hunter, 1995; Uckun et al., 1992; Haimovitz-Friedman et al., 1994).


Uckun et al., (1992) showed that ionizing radiation activated a signal transduction pathway, presumably at the level of a membrane receptor, as evidenced by the enhanced tyrosine phosphorylation of multiple substrates in B-lymphocyte precursors. The protein tyrosine kinase inhibitors genistein and herbimycin prevented radiation-induced tyrosine phosphorylation, DNA fragmentation and cell death suggesting that protein phosphorylation was an important mediator of radiation-induced apoptosis. This pathway was further delineated when it was demonstrated that radiation caused stimulation of phosphatidylinositol turnover; activation of serine/ threonine kinases, including protein kinase C as well as activation of the stress responsive transcription factor NF-kB (Uckun et al., 1993). Uckun et al. (1996) have more recently identified a protein tyrosine kinase, Bruton’s tyrosine kinase (BTK), involved in the process of apoptosis. DT-40 lymphoma B cells, made BTK deficient by targeted disruption of the btk gene, failed to undergo apoptosis after radiation exposure (4 Gy or 8 Gy). On the other hand cells in which the Src protein tyrosine kinase genes lyn or syk were disrupted continued to die by apoptosis post-irradiation. Increased numbers of immature B cells in BTK-deficient mice points to an inability to remove unwanted clones of B cells by apoptosis (Kerner et al., 1996). The mechanism by which ionizing radiation triggers activation of BTK remains unresolved but it is most likely initiated at the level of the membrane. Stevenson et al. (1994) have shown that ionizing radiation and H2O2 are capable of stimulating MAP kinase activity which may account for the activation of early and late-response genes by oxidative stress. It is well established that numerous transcription factors including c-jun, c-fos and NF-kB are activated by UV light and different oxidants (Stein et al., 1989; Devary et al., 1992; Sachsenmaier et al., 1994). In turn a variety of DNA-damage-inducible genes are induced (Fornace, 1992). While there is considerable overlap in the spectrum of genes induced by different DNA damaging agents some genes such as heme oxygenase, tumour necrosis factor a, interleukin I (IL-I), RP2 and RP8 are primarily induced by oxidants such as H2O2 and ionizing radiation (Fornace, 1992). Boothman et al. (1993) have described 12 X-ray-inducible transcripts (Xips), differentially expressed 8–230 fold in irradiated radioresistant melanoma cells. They subsequently observed increased DNA binding activity to oligonucleotide consensus sequences for CREB, NF-kB and Sp1 with extracts from irradiated cells which could account for the alterations in gene expression (Sahijdak et al., 1994). The spectrum of genes induced by ionizing radiation is broad and complex and is responsible for regulating additional transcription events (late response); cell cycle control; proliferation and apoptosis. In some cases such as for NF-kB it has been shown to be involved in protecting against apoptosis (Antwerp et al., 1996; Wang et al., 1996; Wu et al., 1996) while in others, as for p53, it mediates the process of apoptosis in some cell types (Clarke et al., 1993; Lotem and Sachs, 1993; Lowe et al., 1993). RESPONSE THROUGH CERAMIDE As described above radiation-induced signalling is initiated from the plasma membrane via receptors such as EGFR (Knebel et al., 1996). Ionizing radiation mediates its action not only through these membrane receptors but also by activating membrane sphinogomyelinase to cleave sphingomyelin to ceramide and phosphorylcholine (Haimovitz-Friedman et al., 1994). Several cytokines including TNFα signal through the sphingomyelin pathway by activating membrane neutral sphingomyelinase to produce ceramide which in turn activates

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a specific serine/threonine kinase and other intermediates during the process of apoptosis (Obeid et al., 1993; Jarvis et al., 1994). Exposure of bovine endothelial cells to 10 Gy of ionizing radiation caused a rapid degradation of sphingomyelin and a corresponding increase in ceramide (Haimovitz-Friedman et al., 1994). Phorbol ester (TPA) activation of protein kinase C (PKC) has been shown to protect against apoptosis under conditions where ceramide is produced (Obeid et al., 1993; Jarvis et al., 1994). As predicted this compound protected endothelial cells from radiation-induced apoptosis and inhibited production of ceramide. Direct involvement of ceramide in the process was shown by adding C-2 ceramide in the presence of TPA to restore the apoptotic response (Haimovitz-Friedman et al., 1994). Ceramide activates a proline-directed protein kinase which in turn causes phosphorylation of EGFR (Jarvis et al., 1994). Both TNFα and ceramide enhance the phosphorylation of EGFR at Thr 669 (Joseph et al., 1993). The localization of the protein kinase to the membrane and its activation in a cell-free system suggest that it may be a direct target for ceramide (Hannun and Obeid, 1995). Ceramide also activates mitogen-activated protein kinase (MAPK) which suggests that it is an intermediate in the process (Raines et al., 1993). In vitro studies have also demonstrated that ceramide activates ceramide-activated protein phosphatase (CAPP), a member of the PP2A family of ser/thr protein phosphatases (Dobrowsky and Hannun, 1992). The observation that okadaic acid inhibits this enzyme at low concentrations (Dobrowsky and Hannun, 1992) supports results that show this compound interferes with apoptosis induced by ionizing radiation and other agents (Song et al., 1993). Ceramide also has other effects on cell cycle progress inhibiting DNA synthesis and blocking cells in G0/G1 phase, apparently by leading to the dephosphorylation of Rb (Hannun, 1996; Jayader et al., 1995). A direct role in apoptosis stems from the observation that ceramide activates the ICE-like proteases in apoptosis (Smyth et al., 1996; Martm et al., 1995). Treatment of the T-cell leukaemia cell line, CEM, with the cell permeable synthetic ceramide, C-2 ceramide, caused apoptosis and cleavage of fodrin (nonerythroid spectrin) to a 120 kDa fragment (Martin et al., 1995). Recent results demonstrate that fodrin is cleaved both by caspase-3 and calpain during apoptosis (Waterhouse et al., 1997) so it seems likely that ceramide activates these proteases, presumably indirectly to cleave fodrin and other critical substrates. The observation that overexpression of Bcl-2 in leukaemia cells blocked vincristine-induced apoptosis, but not the accumulation of ceramide induced by vincristine, indicates that Bcl-2 functions downstream of ceramide (Zhang et al., 1996). Sensitivity to ionizing radiation has been shown to correlate with the early accumulation of ceramide in a number of different tumour cell lines (Michael et al., 1997). In one radiosensitive Burkitt’s lymphoma line, ceramide increased 4-fold by 10 min post-irradiation (10 Gy) and in the moderately sensitive HL-60 leukaemia cells, ceramide accumulated 2.5fold above basal levels. In contrast, in all radioresistant tumour cells examined, including several Burkitt’s lymphoma lines and a glioma cell line, ceramide did not accumulate postirradiation. The ability to abrogate ceramide production, by pretreatment with the tumour promoter, 12-Otetradecanoylphorbol 13-acetate (TPA), conferred resistance to radiationinduced apoptosis in the sensitive cells. An isogenic subline of the sensitive line was resistant to both C8-ceramide (20 µM) and ionizing radiation-induced apoptosis. Since bypassing the block in radiation-induced ceramide production, by the addition of exogenous ceramide was


not sufficient to induce apoptosis, this suggests the existence of a second ceramide-associated signalling defect in these radioresistant cells which confers resistance to ceramide-induced apoptosis. While many of the effector molecules have been identified, the pathway by which ceramide induces apoptosis remains to be elucidated. CELL CYCLE CHECKPOINT ACTIVATION Exposure of mammalian cells to ionizing radiation causes a delay in progression of cells from G1 into S phase, inhibition of DNA synthesis and a delay in progression from G2 phase into mitosis (Leeper et al., 1972; Konig and Baisch, 1980). In most organisms, DNA damage leads to the rapid and dose-dependent inhibition of DNA synthesis. This dose-dependent inhibition of DNA synthesis appears to have at least two components. One is the blockage of replication by the damage, which has been demonstrated in vivo and in vitro, and appears to be due to a delay in fork progression (Painter, 1985). The other is the blockage of replicon initiation for a period of time. The relative contribution of the two components depends on the type of damage. Yeast mutants defective in checkpoint control either at the G1/S phase or G2/M transitions lose chromosomes spontaneously and are hypersensitive to ionizing radiation and/ or fail to maintain the dependence on completion of prior cell cycle events (Weinert and Hartwell, 1990; Li and Murray, 1991; Jimenez et al., 1992; Al-Khodairy and Carr, 1992). A recent report has shown that Chk1, a putative protein kinase, is a cell cycle transitionspecific effector in the Rad3-dependent DNA damage pathway (Walworth and Bernards, 1996). Rad3 is a member of the phosphatidylinositol 3-kinase family and thus related to the ATM gene mutated in ataxia-telangiectasia and like this gene a non-essential gene. Another gene, MEC1 from Saccharomyces is required for both the G1/S and G2/M checkpoints and meiotic recombination (Weinert et al., 1994). Cells expressing a partial loss-of-function allele for this gene fail to inhibit DNA replication after exposure of cells to radiation, reminiscent of A-T cells (Paulovich and Hartwell, 1995). Overexpression of the RAD53/ SAD1 checkpoint kinase suppresses mec 1 lethality in S.cerevisiae (Sanchez et al., 1996). RAD53 encodes a protein kinase and is required for the G1/S and G2/M damage checkpoints, control of DNA replication and transcriptional response to DNA damage (Weinhert and Hartwell, 1988). Exposure of eukaryotic cells to ionizing radiation leads to the activation of several checkpoints that control the progression of cells between different phases of the cell cycle (Kastan et al., 1991). Progression of cells from G1 phase to S phase is normally controlled by cyclin-dependent kinases (cyclin E—cdk 2 and cyclin D1-cdk4), the phosphorylation status of these kinases, the substrates of these kinases such as the retinoblastoma protein and cyclin-kinase inhibitors (Hartwell and Kastan, 1994; Pardee, 1989). Signals arising due to DNA damage activate the product of the tumour suppressor gene p53 to prevent the progression of cells from G1 to S phase (Kastan et al., 1991; Pardee et al., 1989).This is achieved by p53-activated induction of the cdk inhibitor p21/WAF1 which binds to and inhibits cyclin-kinase complexes preventing the phosphorylation of specific substrates and in turn the passage of cells from G1 to S phase (Harper et al., 1993; El-Deiry et al., 1993; Xiong et al., 1993). Wild type p53 blocks progression of cells from G1 to S phase in response to DNA damage (Kastan et al., 1991; Kastan, 1993) or when it is overexpressed in

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cells (Zambetti and Levine, 1993). After exposure of cells to DNA damaging agents, p53 is translocated from the cytoplasm to the nucleus where it can bind to specific DNA sequences to positively or negatively regulate transcription (El-Deiry et al., 1993). After treatment with radiation and other agents the half-life of p53 increases from 10–30min to several hours (Reich and Levine, 1984). Since p53 has multiple phosphorylation sites it has been suggested that phosphorylation is the means of stabilization (Ullrich et al., 1992a,b). When rat embryo fibroblasts transfected with activated ras are exposed to phorbol ester cooperation with wild type p53, to bring about growth arrest, is observed (Delphin and Baudier, 1994). Under these conditions a specific enhancement of wild type p53 phosphorylation occurs and there is an enhancement of p53-DNA binding. The PKC mode of phosphorylation in vitro also stimulates p53 DNA-binding activity. Delphin and Baudier (1994) propose that PKC and p53 participate in a negative feedback control of phosphoinositide signals common to mitogenic stimulation. Wild type p53 leads to either G1 arrest or apoptosis in different cell types, but no G1 arrest is observed with mutant p53 and apoptosis can be prevented (Kastan et al., 1991; Lotem and Sachs, 1993). ATAXIA-TELANGIECTASIA The enhanced chromosomal aberrations and the reduced survival of A-T cells after exposure to ionizing radiation suggests that the defect in A-T cells is due to a deficiency in the removal of radiation damage in DNA (Chen et al., 1978; Taylor et al., 1975). Paterson et al., (1976) reported that some A-T cell lines had a reduced capacity to remove thymine glycol damage from DNA at high radiation doses. Several other studies failed to reveal a defect in the repair of single strand breaks or other forms of damage in DNA (Fornace and Little, 1980; Lavin and Davidson, 1981; Lehmann et al., 1982; Shiloh et al., 1983; Taylor et al., 1975). Considerable effort has been expended to determine whether the sensitivity to radiation in A-T is associated with defective double-strand break repair. A variety of earlier reports utilizing neutral sucrose gradients, neutral elution and pulsefield gel electrophoresis failed to reveal any significant defect in double-strand break rejoining at short times after irradiation (Blocher et al., 1991; Lehmann and Stevens, 1979; Taylor et al., 1975; Van der Schans, 1982). However, Coquerelle and Weibezahn (1981) and Coquerelle et al., (1987) reported that rejoining of γ-radiation-induced DNA double-strand breaks was slower in A-T fibroblasts than in controls. More support for a defect in double-strand break rejoining was provided by Cornforth and Bedford (1985) when they demonstrated the existence of residual breaks in A-T cells utilizing the premature chromatin condensation assay. Such hypersensitivity would be explained by the continued existence of 5–10% of the total breaks induced. Now that we know the identity of the A-T gene, ATM (Savitsky et al., 1995), its possible mode of action in cell cycle control and in detection of DNA damage (Lavin and Shiloh, 1996; Lavin and Shiloh, 1997) another explanation is possible. If ATM is a “sensor” of DNA damage then it is likely that inability to recognize the damage is responsible for the residual DNA breaks and their translation into chromosome breaks rather than a repair defect per se. Abnormalities in chromatin structure might be indirect due to absence of ATM or the presence of a mutated form of ATM. Either way the residual chromosome


breaks may be at least partially responsible for the radiosensitivity in A-T and may also contribute to the genome instability and cancer predisposition. A-T cells are characterized by a failure to activate either the Gl/S or G2/M checkpoints at short times after radiation exposure and they exhibit radioresistant DNA synthesis (Beamish and Lavin, 1994; Houldsworth and Lavin, 1980; Nagasawa and Little, 1983; Painter and Young, 1980; Scott and Zampetti-Bosseler, 1982). The inability of A-T cells to activate these checkpoints efficiently ultimately leads to the accumulation of irradiated cells in G2/M phase where they die. The molecular nature of the defect in cell cycle control in A-T cells has been extensively studied and it is evident that a pathway mediated by the ATM gene functioning through p53 is defective in A-T cells (Artuso et al., 1995; Kastan et al., 1992; Keegan et al., 1996; Khanna and Lavin, 1993; Lu and Lane, 1993; Mirzayans et al., 1995). The defect is evident not only at the level of radiation-induced stabilization of p53 but also at the level of its downstream effectors WAF1, gadd 45 and MDM2 (Canman et al., 1994; Khannaila et al., 1995). Increased WAF1, as a consequence of exposure of control cells to radiation leads to an inhibition of cyclin-dependent kinase activity (cyclinE cdk2 at the G1/S transition point) (Dulic et al., 1994). This inhibition is caused by a change in the stoichiometry of binding of WAF1 to a complex that includes cyclinE, cdk2, PCNA and WAF1 (Xiong et al., 1993). Inhibition of cyclinE-cdk2 activity prevents phosphorylation of substrates required for entry into S phase causing G1 arrest. These substrates include the retinoblastoma protein, which when phosphorylated disassociates from the transcription factor E2F and allows for the transcription of key enzymes such as thymidine kinase and ribonucleotide reductase necessary for S phase. In A-T cells WAF1 does not increase in response to radiation or is delayed in its induction, consequently, inhibition of cyclin E— cdk2 is not observed and cells progress from G1 into S phase without delay (Khanna et al., 1995). Lack of inhibition of cyclin E—cdk2 in irradiated A-T cells appears to be due to little or no change in WAF1 association with the kinase (Beamish et al., 1996). However, since the S phase and G2/M checkpoints are also defective in A-T one might expect some overlap given the likely involvement of ATM in control of the other checkpoints. Resistance of several different cyclin-dependent kinase activities, that control the different checkpoints, to radiation-induced inhibition in A-T cells points to a wider role for the p53 pathway in radiation signal transduction (Beamish et al., 1996). The failure of A-T cells to mount a p53 response effective in delaying the progression of cells from G1 to S phase is unlikely to provide an explanation for the enhanced radiosensitivity in these cells. Others have not observed a correlation between absence of p53 or mutated (non-functional) p53 and propensity to radiation sensitivity (Clarke et al., 1993; Lee and Bernstein, 1993; Lowe et al., 1993; Slichenmyer et al., 1993). It is more likely that the defective p53 response in G1 phase cells leads to increased chromosomal instability as a consequence of these cells entering S phase prior to repairing damage in DNA. Indeed chromosomal instability and predisposition to develop leukaemias, lymphomas and to a lesser extent solid tumours are very characteristic of A-T (Boder and Sedgwick, 1963; Hecht and Hecht, 1990; Spector et al., 1982). Some evidence has been provided that p53 may also play a role in other checkpoints in S phase and G2/M (Agarwal et al., 1995; Aloni-Grinstein et al., 1994; Beamish et al., 1996). Flow cytometric analysis has shown that when A-T cells are irradiated in G1 or S phase and allowed to proceed through the cycle they accumulate essentially, irreversibly in the following G2/M where they die (Beamish and Lavin, 1994). These results suggest that by

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ignoring the cell cycle checkpoints, A-T cells ultimately carry chromosome damage into G2 phase where they are incapable of normal chromosomal segregation and cell division. Treatment of irradiated A-T cells with caffeine allows them to bypass G2 delay but leads to massive chromosomal fragmentation and cell death, supporting the hypothesis that A-T chromosomes accumulate damage and as a consequence are incapable of proceeding to mitosis (Bates et al., 1985). Further elucidation of the role of the ATM protein will provide a greater understanding of the basis for radiosensitivity in A-T. REFERENCES Agarwal, M.L., Agarwal, A., Taylor, W.R. and Stark, G.R. (1995) p53 controls both the G2/M and the G1 cell cycle checkpoints and mediates reversible growth arrest in human fobroblasts. Proc. Natl. Acad. Sci. USA, 92, 8493–8497. Al-Khodairy, F. and Carr, A.M. (1992) DNA repair mutants defining G2 checkpoint pathways in Schizosaccharomyces pombe. EMBO J., 11, 1343–1350. Alnemri, E.S., Livingston, D.J., Nicholson, D.W., Salvesen, G., Thornberry, N.A., Wong, W.W. and Yuan, J. (1996) Human ICE/CED-3 protease nomenclature. Cell, 87, 171. Aloni-Grinstein, R., Schwartz, D. and Rotter, V. (1994) Accomodation of wild-type p53 protein upon γ-irradiation induces a G2 arrest-dependent immunoglobulin k light chain gene expression. EMBO J., 14, 1392–1401. Artuso, M., Esteve, A., Bresil, H., Vuillaume, M. and Hall, J. (1995) The role of the ataxiatelangiectasia gene in the p53, WAF1/CIP1(p21) GADD45-mediated response to DNA damage produced by ionizing radiation. Oncogene, 8, 1427–1435. Badie, C.G., liiakis, N., Foray, G., Alsbeih, B., Pantelias, R., Okayasu, N., Cheong, N.S., Russel, A.C., Begg, C.F., Arlett, C.F. and Malaise, E.P. (1995) Defective repair of DNA doublestrand breaks and chromosome damage in fibroblasts from a radiosensitive leukemia patient. Cancer Res., 55, 1232–1234. Bates, P.R., Imray, F.P. and Lavin, M.F. (1985) Effect of caffeine on γ-ray induced G2 delay in ataxia telangiectasia. Int. J. Radiat. Biol., 47, 713–722. Baxter, G.D., Collins, R.J., Harmon, B.V., Kumar, S., Prentice, R.L., Smith, P.J. and Lavin, M.F. (1989) Cell death by apoptosis in acute leukaemia. J. Pathol., 158, 123–129. Baxter, G.D. and Lavin, M.F. (1992) Specific protein dephosphorylation in apoptosis induced by ionizing radiation and heat-shock in human lymphoid cells. J. Immunol., 148, 1949–1954. Beamish, H. and Lavin. M.F. (1994) Radiosensitivity in ataxia-telangiectasia: anomalies in radiationinduced cell cycle delay. Int. J. Radiat. Biol., 65, 175–184. Beamish, H., Williams, R., Chen, P. and Lavin, M.F. (1996) Defect in multiple cell cycle checkpoints in ataxia-telangiectasia post-irradiation. J. Biol. Chem., 271, 20486–20493. Biedermann, K.A., Sun, J-R., Giaccia, A.J.,Tosto, L.M. and Brown, J.M. (1991) SCID mutation in mice confers hypersensitivity to ionizing radiation and a deficiency in DNA double-strand break repair. Proc. Natl. Acad. Sci. USA, 88, 1394–1397. Blocher, D., Sigut, D. and Hannan, M.A. (1991) Fibroblasts from ataxia-telangiectasia (A-T) and AT heterozygotes show an enhanced level of residual DNA double-strand breaks after low doserate γ-radiation as assayed by pulsed field gel electrophoresis. Int. J. Radiat. Biol., 60, 791–802. Blok, J. and Lohman, H. (1973) The effects of γ-radiation in DNA. Current Topics in Radiation Research, 9, 165–245.


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Part 2 REGULATION OF APOPTOSIS

4. KINASE CASCADES AND APOPTOSIS J.F.HANCOCK*† *Dept. of Pathology, University of Queensland Medical School, Herston Road, Brisbane, QLD 4006

KEY WORDS: stress activated protein kinase, mitogen activated protein kinase, phosphoinositide 3-OH kinase, Akt, protein kinase B.

INTRODUCTION There are a number of extracellular signals or insults that induce apoptosis and a variety of growth factors and cytokines that can promote cell survival in the presence of an apoptotic signal. The cell execution machinery and the immediate upstream regulators of this machinery are rapidly being unravelled. An important question then, is what signal transduction pathways link membrane events to the apoptotic effector machinery? There is good evidence that stress activated protein kinases (SAPKs) are activated by signals that induce apoptosis and that constitutive activation of the SAPKs can induce apoptosis. It is also clear that activation of phosphoinositide 3-OH kinase (PI3K) and the Akt serine/threonine kinase can promote cell survival in the presence of stimuli that normally induce apoptosis. A similar protective role for the mitogen activated protein kinase (MAPK) cascade has been proposed. While it is clear that these well defined signalling cascades play a role in regulating apoptosis, the critical targets of these cascades are largely unknown, in other words, the wiring diagrams connecting to the effectors of apoptosis are incomplete. The aim of this chapter is to briefly review the organization and mechanisms of activation of the PI3K/ Akt, MAPK and SAPK pathways and discuss some of the data that implicate these pathways in signalling for cell survival or apoptosis. Other pathways that signal for apoptosis are reviewed elsewhere in this book.

† Corresponding Author: Tel.: j.hancok@Mailbox. uq.edu.qu

+61–7–3365–5340,

Fax:

+61–7–3365–5511,

e-mail:

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Figure 4.1 Activation of phosphoinositide 3-OH kinase and Akt/PKB. Cartoon of the critical events leading to the activation of the serine/threonine kinase Akt/PKB by upstream kinase (USK) and D3-phosphorylated phosphatidylinositides generated within the inner leaflet of the plasma membrane by PI3K.

PHOSPHOINOSITIDE 3-OH KINASE AND AKT Mechanism Of Activation Of PI3K And Akt Recruitment and activation of phosphoinositide 3-OH kinase (PI3K) occurs when ligands including EGF, PDGF, bFGF, IGF-1 and insulin bind to their cognate receptor tyrosine kinases (Figure 4.1). PI3K is a heterodimer comprising a noncatalytic p85 subunit and a catalytic p110 subunit, both of which have several isoforms (Carpenter and Cantley, 1996). The SH2 domains of the p85 subunit recruit PI3K to the plasma membrane by binding to specific phosphotyrosine docking sites, either on the C-terminal tail of the receptor tyrosine kinase or on the IRS-1 adapter protein. The docking of p85 onto phosphotyrosine residues partially activates the associated p110 catalytic subunit, but additional activating inputs are provided by Src and Ras (Rodriguez Viciana et al., 1994.; Carpenter and Cantley, 1996; Rodriguez Viciana et al., 1996). Activated PI3K phosphorylates the D3 position of inositol phospholipids. A major in vivo substrate of PI3K is phosphatidylinositol 4–5 bisphosphate (PtdIns 4, 5 P2), which is converted to PtdIns 3, 4, 5 P3. A phospholipid 5’-phosphatase then generates PtdIns 3, 4 P2. PtdIns 3, 4, 5 P3 and PtdIns 3,4 P2 bind pleckstrin homology (PH) domains with high affinity and so the generation of these PtdIns in the inner leaflet of the plasma membrane provides novel docking sites for signalling molecules that contain PH domains (Hemmings, 1997b). Two such molecules are the Akt serine/threonine kinase (also

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called Protein Kinase B, PKB) (Bos, 1995; Burgering and Coffer, 1995; Datta et al., 1996) and its activating kinase, PKBK (PKB kinase; also called Upstream Kinase, USK). Akt binds to PtdIns 3, 4, 5 P3 or PtdIns 3,4 P2 in the inner leaflet the plasma membrane via its PH domain (Franke et al., 1995; James et al., 1996; Franke et al., 1997; Hemmings, 1997a; Klippel et al., 1997). The interaction between the Akt PH domain and PtdIns 3, 4, 5 P3 or PtdIns 3, 4 P2 induces a conformational change in the protein that allows PKBK to phosphorylate T308 and partially activate Akt (Alessi et al., 1997; Stokoe et al., 1997). The binding of PtdIns 3, 4 P2 also induces Akt dimerization, which may contribute to activation (Franke et al., 1997). Full activation of Akt requires phosphorylation on S473 of the partially activated molecule (Alessi et al., 1997; Hemmings, 1997c; Stokoe et al., 1997). Although the in vivo S473 protein kinase has not yet been identified, it is interesting to note that MAPKAP-K2, a SAPK2 target, can phosphorylate S473 and partially activate Akt in vitro (Alessi et al., 1996). Activated Akt plays a critical role in signalling for cell survival through as yet unidentified signalling pathways; these data will be examined in more detail in the next section. In addition Akt can directly phosphorylate and inactivate GSK3, thus stimulating glycogen synthesis (Cross et al., 1995; Cross et al., 1997), and indirectly activate p70S6K, thus upregulating protein synthesis (Burgering and Coffer, 1995). Other roles of PI3K include: regulation of the actin cytoskeleton and activation of Rac, most likely through the plasma membrane recruitment of a Rac exchange factor containing a PH domain (Kotani et al., 1994; Wennstrom et al., 1994; Hawkins et al., 1995; Nobes and Hall, 1995; Rodriguez Viciana et al., 1997). In addition, certain calcium independent, atypical isoforms of protein kinase C are activated by PtdIns 3, 4 P2 and PtdIns 3, 4, 5 P3 (Carpenter and Cantley, 1996). Role Of PI3K And Akt/PKB In Anti-Apoptotic Signalling The PI3K/Akt pathway is now firmly established as an important cell survival signalling pathway. The story has evolved rapidly over the past few years, having started with observations that insulin-like growth factor 1 (IGF-1) and PDGF promote cell survival in the presence of diverse apoptotic stimuli and progressed most recently to identification of Akt/ PKB as the critical effector. Identification of IGF-1 and PDGF as survival factors Hemopoietic growth factors promote survival of progenitor cells by preventing apoptosis, however, IGF-1 can partially replace the requirement for such factors. For example, human erythroid precursors growing in vitro rapidly undergo apoptosis when switched to serum free medium. If erythropoietin is included as the sole growth factor, >75% of the cells survive, while inclusion of IGF-1 as sole growth factor rescues >50% of the cells (Muta and Krantz, 1993). Similarly, IGF-1 promotes the proliferation and survival of IL3-dependent FDCP-1/Mac-1 murine myeloid progenitors in the absence of exogenous IL3 (Minshall et al., 1996). Thus IGF-1 partially protects against apoptosis induced by the withdrawal of growth factors from haemopoietic cell lines. Similar observations have been made for IGF-1 and apoptosis induced by chemotherapeutic drugs. For example, mouse BALB/c 3T3 cells overexpressing IGF-1-R arrest in S phase and undergo apoptosis when treated with the

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topoisomerase I inhibitor etoposide. However, when grown in the presence of IGF-1, these cells still arrest in S phase when exposed to etoposide but are resistant to apoptosis. The survival effect of IGF-1 requires a functional IGF-1-R because IGF-1 provides minimal protection to parental BALB/c 3T3 cells expressing wild type levels of IGF-1-R and has no protective effect on fibroblasts with a targeted disruption of the IGF-1-R locus (Sell et al., 1995). Deregulated expression of the c-myc oncogene induces apoptosis in serum starved fibroblasts (Evan et al., 1992). However, cells overexpressing c-myc are protected from apoptosis when grown in low serum containing IGF-1 or PDGF (Harrington et al., 1994). IGF-1 also functions as an in vitro survival factor for primary cultures of cerebellar neurons (Dudek et al., 1997) and fibroblasts exposed to UV irradiation (Kulik et al., 1997). The survival effect of IGF-1 is independent of its proliferative action since it protects cells both before and after the G1/S checkpoint(Harrington et al., 1994; Sell et al., 1995). PI3K mediates cell survival Many studies have now shown that the survival signalling pathway downstream from the IGF-1 receptor requires PI3K. For example, primary cultures of cerebellar neurons maintained in IGF-1 have elevated PI3K activity and undergo apoptosis when treated with the PI3K inhibitors wortmannin or LY294002 (Dudek et al., 1997). Rat-1 cells growing in IGF-1 are protected against apoptosis induced by UV-B: the protection is lost when the cells are treated with wortmannin but is unaffected when the cells are treated with the MEK inhibitor PD98059 (Kulik et al., 1997). In addition, Rat-1 cells that express constitutively active PI3K are protected against UV-B to the same extent as those grown in IGF-1, but this protection remains wortmannin sensitive (Kulik et al., 1997). Kauffmann Zeh et al., (1997) found that expression of activated Ras sensitizes fibroblasts to myc-induced apoptosis and used Ras effector site mutants to identify the effector pathway responsible. A Ras effector mutant that activates only the Raf/ MEK/MAPK cascade sensitizes, and a Ras effector site mutant that activates only PI3K protects against mycinduced apoptosis. A Ras effector mutant that activates only RalGDS does not protect or sensitize. Consistent with these results, expression of constitutively activated PI3K also protects fibroblasts from apoptosis induced by myc-expression (Kauffmann Zeh et al., 1997). Other data show that PI3K is also the critical mediator of NGF-induced cell survival. Nerve growth factor (NGF) induces both differentiation and survival of neuronal cells. These two biological effects of NGF are mediated by different signalling pathways: NGF induced differentiation of the PC12 pheochromocytoma cell line requires the Ras/Raf/ MEK/MAPK pathway (Cowley et al., 1994; Marshall, 1995; Yao and Cooper, 1995), while Ras is not required for the NGF-mediated survival of PC12 cells in serum-free medium (Yao and Cooper, 1995). The survival effect of NGF is completely abrogated by the PI3K inhibitors wortmannin or LY294002. Heterologous expression of wild type PDGFR in PC12 cells renders them sensitive to the survival effect of PDGF. However, PC12 cells expressing a mutant PDGFR, which is unable to bind and activate PI3K, are not protected from undergoing apoptosis by PDGF. Taken together these data strongly implicate PI3K as a critical signalling protein in NGF and PDGF induced cell survival (Yao and Cooper, 1995).

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Before examining events downstream of PI3K, it is worth noting that survival signalling is not all mediated through PI3K. For example, IGF-1 and IL3 both efficiently promote survival of IL3-dependent murine myeloid precursors and both activate PI3K (Minshall et al., 1996). Cells maintained in IGF-1, but not those in IL3 undergo apoptosis when treated with wortmannin at concentrations that completely block PI3K activity. Thus in addition to the PI3K mediated survival pathway, IL3 must activate a second PI3K independent survival pathway in hemopoietic cells (Minshall et al., 1996). The second IL3 pathway remains to be elucidated, but as for the PI3K mediated pathway (Harrington et al., 1994; Yao and Cooper, 1995; Kulik et al., 1997) it does not require new protein synthesis (Minshall et al., 1996). Akt/PKB is the critical PI3K target As discussed above there are three kinases known to be downstream of PI3K. Akt/ PKB and USK are direct targets of PI3K, while GSK3 and p70S6K are, respectively, direct and indirect targets of Akt/PKB. PI3K undoubtedly has multiple other targets, i.e. any protein with a PH domain. Nevertheless, attention has been sharply focused on whether Akt/PKB and p70S6K are the PI3K targets involved in signalling for cell survival. PC12 cells growing in NGF, or Rat-1 cells growing in serum, both undergo apoptosis when treated with the PI3K inhibitors wortmannin and LY294002, but are unaffected by treatment with rapamycin at concentrations that demonstrably inhibit p70S6K activation (Yao and Cooper, 1996). The survival of primary cultures of cerebellar neurons, which is promoted by IGF-1 and blocked by wortmannin and LY294002, is not compromised by rapamycin (Dudek et al., 1997). Similarly the IGF-1 mediated survival of Rat-1 cells exposed to UV-B, and the PI3K mediated survival of c-myc expressing fibroblasts are unaffected by rapamycin (Kauffmann Zeh et al., 1997; Kulik et al., 1997). It is clear from these studies that p70S6K is not involved in signalling for cell survival. In contrast, Akt constitutively targeted to the plasma membrane by myristoylation protects COS 7 cells against apoptosis induced by UV-B (Kulik et al., 1997). Similarly, a gag-Akt fusion protein, constructed to mimic v-Akt, the constitutively active retroviral protein, protects fibroblasts against myc-induced apoptosis, while a kinase inactive gag-Akt fusion renders no protection. The protection afforded by the gag-Akt fusion protein is insensitive to LY294002, demonstrating that Akt is downstream from PI3K (Kauffmann Zeh et al., 1997). Akt is also activated by IGF-1 in cerebellar neurons and overexpression of wild type Akt in cerebellar neurons is sufficient to promote their survival in the absence of IGF-1. Furthermore, expression of dominant negative forms of Akt in cerebellar neurons abrogates the anti-apoptotic effect of IGF-1 (Dudek et al., 1997). Essentially identical observations have recently been made in epithelial cells induced to undergo apoptosis by detachment from the extracellular matrix (so called anoikis). Activated Ras, activated PI3K and activated Akt all protect epithelial cells against anoikis. The protection afforded by Ras and PI3K, but not that afforded by Akt is wortmannin sensitive (Khwaji et al., 1997). Taken together these data clearly implicate PI3K mediated activation of Akt as an important anti-apoptotic signalling pathway. What then are the critical targets of Akt that regulate the apoptotic effector machinery? Since p70S6K clearly plays no role and GSK3 has never been implicated in apoptotic control, the answer must await the identification of novel Akt targets. However, somewhat intriguingly, a recent study has shown that IGF-1

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can suppress apoptosis caused by overexpression of interleukin-1β-converting enzyme (Jung et al., 1996). The study did not address the signalling pathways involved, other than demonstrating that IGF-1 did not alter the expression of Bcl-2, Bcl-x, or Bax. In the light of the data discussed here, it is tempting to implicate Akt as the critical effector for this IGF-1 survival effect, which opens up the possibility that Akt may, inter alia, directly regulate a caspase cascade. MAP KINASE CASCADES Organization Of The MAPK Cascade This conserved signalling pathway, delineated by genetic experiments in invertebrates and biochemistry in mammalian cells, comprises a basic module of Raf->MAPKKinase (MKK)>MAPK with tyrosine kinases and Ras as upstream activators (Figure 4.2). A large number of growth factors and cytokines, e.g. EGF, PDGF, FGF, Insulin, GM-CSF, M-CSF, IL3, activate this module in conjunction with other signalling pathways. In addition to being required and sufficient for DNA synthesis, activation of the MAPK cascade may promote cell survival and depending on the cellular context, stimulate cell differentiation. MAPK activation occurs rapidly after the engagement of receptor by ligand but the duration of activation varies for different ligands. The critical initiating event in MAPK activation is recruitment of the Ras exchange factor hSos1 to the plasma membrane (Aronheim et al., 1994) where Ras is localized; hSos1 then activates Ras by catalysing the exchange of GDP for GTP (Chardin et al., 1993). The exchange factor hSos1 is recruited to the plasma membrane in a complex with adapter proteins Grb2 or Grb2/Shc (Buday and Downward, 1993; Egan et al., 1993; Li et al., 1993).The adapter proteins bind to specific phospho-tyrosine residues on the cytoplasmic tail of activated tyrosine kinases. The kinases may be the growth factor receptor per se, or in the case of cytokine receptors, a recruited cytoplasmic non-receptor tyrosine kinase. Recently, a membrane protein, FRS2, which is tyrosine phosphorylated following FGF stimulation, has been shown to be an alternative plasma membrane docking site for Grb2/ Sos complexes (Kouhara et al., 1997). Once RasGTP is generated at the plasma membrane multiple effector proteins are activated, including Raf (Moodie et al., 1993), PI3K (RodriguezViciana et al., 1994), RalGDS(Kikuchi et al., 1994) and AF6 (Kuriyama et al., 1996). There are three Raf serine threonine kinases, c-Raf-1, A-Raf and B-Raf (Daum et al., 1994). c-Raf-1 is widely expressed while A-Raf and B-Raf have more restricted expression patterns. The mechanism of activation of the Raf kinases is not fully resolved, but is best worked out for c-Raf-1. RasGTP recruits Raf from the cytoplasm to the plasma membrane by binding the N-terminal CR1 domain of Raf (Traverse et al., 1993; Van Aelst et al., 1993; Vojtek et al., 1993; Zhang et al., 1993). This recruitment of Raf to the plasma membrane is sufficient for partial activation (Leevers et al., 1994; Stokoe et al., 1994; Wartmann and Davis, 1994). Once at the plasma membrane Raf is available for phosphorylation by tyrosine kinases on residues Y340 and Y341 which leads to further activation (Fabian et al., 1993; Marais et al., 1995). Ras also has an additional role in potentiating the activity of tyrosine

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Figure 4.2 Activation of MAPK and SAPK cascades. Overview of the organization of the MAPK and SAPK cascades. Question marks indicate “missing kinases”, that have not yet been identified, or activation mechanisms that have not been elucidated. Alternative names of the various kinases are given in the text.

phosphorylated Raf through an interaction with the Raf zinc finger (Jelinek et al., 1996; Luo et al., 1997; Roy et al., 1997). Other activating inputs into Raf may include: phosphatidylserine binding to the Raf zinc finger (Ghosh et al., 1996), displacement of 14–3– 3 from the Raf N-terminus (Rommel et al., 1996), phosphorylation of T269 by the ceramide activated protein kinase, Ksr 1 (Yao et al., 1995; Zhang et al., 1997), phosphorylation of S499 by PKC (Kolch et al., 1993) and potentially Raf dimerization (Farrar et al., 1996; Luo et al., 1996). The relative contribution of each of these mechanisms to A-Raf and B-Raf activation is probably different from c-Raf-1 since both of these Raf isoforms have negatively charged residues in place of one or both regulatory tyrosines. The presence of these negatively charged residues significantly increases the basal kinase activity of A-Raf and BRaf but A-Raf and B-Raf still exhibit Ras dependent activation following membrane recruitment (Marais et al., 1997). Recent experiments have also defined an alternative mechanism for the activation of B-Raf in PC12 cells, that is stimulated by cAMP and mediated through the Ras subfamily GTPase, Rap1 (Vossler et al., 1997). These in vivo data are consistent with earlier observations that showed activation of a B-Raf/14–3–3 complex by GTP loaded Rap 1 in a cell free system (Ohtsuka et al., 1996). All Raf kinases activate the dual specificity kinase MEK1 (MAP Kinase Kinase 1, MKK1) by phosphorylating residues S218 and S222 (Dent et al., 1992; Kyriakis et al., 1992; Alessi et al., 1994; Zheng and Guan, 1994). c-Raf-1 but not A-Raf also activates the closely related

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dual specificity kinase MEK2 (MKK2) (Wu et al., 1996). MKK1 and MKK2 in turn activate p44 MAPK1 (ERK1) and p42 MAPK2 (ERK2) by phosphorylating the threonine and tyrosine residues within a regulatory TEY motif in the kinase domain (Crews et al., 1992; Howe et al., 1992; Macdonald et al., 1993; Marshall, 1994). Activated MAPK1 and MAPK2 have a plethora of substrates which include transcription factors, e.g. Elk1 (Janknecht et al., 1993), other kinases p90rsk (MAPKAP-K1) (Blenis, 1993), signal transduction molecules, e.g. Sosl, MEK1, cPLA2 (Lin et al., 1993; Cadwallader et al., 1994; Buday et al., 1995), and regulators of translation. The MAPK phosphorylations either activate or downregulate the activity of each substrate. The net effect of MAPK activation depends on both the cell type and the duration of activation (Marshall, 1995). For example, in NIH3T3 fibroblasts activation of the Raf/MEK/MAPK cascade is sufficient to stimulate DNA synthesis and cell division (Cowley et al., 1994; Mansour et al., 1994). In neuronal PC12 cells transient MAPK activation as stimulated by EGF results in cell division, while sustained activation as stimulated by NGF is accompanied by nuclear translocation and results in differentiation (Cowley et al., 1994; Marshall, 1995). The downregulation of the Raf/MEK/MAP kinase pathway is less well understood. MAPK phosphorylation of the C-terminus of hSos1 may cause dissociation of the Grb2/Shc/ EGFR complex (Buday et al., 1995; Porfiri and McCormick, 1996), so terminating the activation of Ras. Ras GAPs compete with Ras effectors for binding to RasGTP and when bound, catalyse rapid GTP hydrolysis, returning Ras to the inactive ground state (McCormick et al., 1988; McCormick, 1989). It is unclear, however, how activated Raf, that is no longer complexed with Ras at the plasma membrane, is deactivated and returned to the cytoplasm. There is clearly a role for phosphatases (Dent et al., 1995; Dent et al., 1996) and possibly also for 14–3–3 proteins. Organization Of The SAPK Cascades There are four Stress Activated Protein Kinases (SAPKs) that are activated by cellular stress: heat and osmotic shock, protein synthesis inhibition, DNA damaging agents, UV radiation and proinflammatory cytokines IL1 and TNF (Kyriakis and Avruch, 1996). The basic module is SAPK Kinase Kinase (SAPKKK)->SAPK Kinase (SKK, also frequently referred to as MKK)->SAPK, but the details of how specific stimuli activate the various SAPK cascades is not nearly as well worked out as the MAPK cascade (Figure 4.2). The nomenclature of the SAPK cascades suffers from multiple designations for all of the kinases; in this discussion alternate names will usually be given only once, when first mentioned. SAPK1 (also called Jun N-terminal Kinases, JNKs) and SAPK2a (also called: RK, p38, CSBP kinase, Mxi2) activation is under the control of the Rho family GTPases Rac and Cdc42 (Coso et al., 1995; Minden et al., 1995; Olson et al., 1995). The details of how Rho family GTPases are activated are not yet worked out. Extrapolating from the regulation of Ras by hSos1, it is postulated that colocalizing Rac and Cdc42 with their cognate exchange factors causes activation (van Leeuwen et al., 1995; Michiels et al., 1997). Several exchange factors have been cloned, including Dbl (Hart et al., 1991) and other proteins containing Dbl-like homology domains: Vav (Katzav et al., 1989; Adams et al., 1992), Tiam2 (Habets et al., 1994), Isc (Whitehead et al., 1996) and possibly hSos1 (Chardin et al., 1993). How these exchange factors are recruited to the plasma membrane to activate Rac or Cdc42 is unclear.

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However, the presence of pleckstrin homology domains in many Dbl-like exchange factors suggests that PtdIns 3, 4 P2 or PtdIns 3, 4, 5 P3 might constitute the membrane docking site (Zheng et al., 1996; Michiels et al., 1997; Wang et al., 1997). Consistent with this model, it has recently been shown that Rac activation is downstream of PI3K (Hawkins et al., 1996), but other mechanisms must also operate, because tyrosine phosphorylation is required to activate the Vav exchange factor(Hanrto et al., 1997). The functional homologue of Raf in the MAPK cascade, is MEKK1, and as with Raf the regulation of MEKK1 is not fully resolved. In vitro, MEKK1 can bind both Ras and the Rho subfamily GTPases, Rac and Cdc42 (Lange Carter and Johnson, 1994; Russell et al., 1995), however, in vivo, activated Rac and Cdc42 robustly activate the SAPK1 cascade while Ras does so weakly (Coso et al., 1995; Minden et al., 1995; Olson et al., 1995). To add further complexity, there are at least three other isoforms of MEKK (MEKK2–4) with near identical kinase domains (96% conserved), which all activate SAPK1 when conditionally expressed (Ellinger Ziegelbauer et al., 1997; Gerwins et al., 1997). The structure of their regulatory domains varies and only MEKK1 and MEKK4 can bind Rac and Cdc42, thus each MEKK may be subject to different mechanisms of regulation and transduce distinct upstream signals (Fanger et al., 1997). MEKK1–4 all activate the dual specificity kinase MKK4 (MAPK Kinase 4, SKK1 or SEK1, SAPK Kinase 1) and when overexpressed can activate MKKl(Lange Carter et al., 1993; Minden et al., 1994; Yan et al., 1994). In addition, MEKK1 is an IκB kinase capable of triggering IκB degradation and NF-κB activation (Liu et al., 1996; Lee et al., 1997). Thus signals which activate MEKK1 will coordinately activate c-Jun and NF-κB dependent gene transcription. Other SAPKKKs with uncharacterized upstream activators include, mixed lineage kinase (MLK-3) which activates MKK4 and possibly MKK3/MKK6 and Tp12 which also activates MKK4 (Salmeron et al., 1996; Tibbles et al., 1996). Germinal center kinase is a B cell specific Ste20 homologue that activates MKK4/SAPK1 via an as yet unidentified SAPKKK (Pombo et al., 1995). MKK4 activates SAPK1 by phosphorylating tyrosine and threonine residues in the TGY regulatory sequence, which is a signature motif of all SAPKs (Sanchez et al., 1994; Yan et al., 1994). There are three closely related isoforms of SAPK1 (JNKs) which phosphorylate c-Jun on residues S63 and S73 and upregulate its transcriptional activity (Derijard et al., 1994; Kyriakis et al., 1994). They also phosphorylate ATF2 (Gupta et al., 1995; van Dam et al., 1995; Beyaert et al., 1996; Hazzalin et al., 1996). The dual specificity kinase MKK3 (SKK2) is a specific activator of SAPK2a while the dual specificity kinase MKK6 (SKK3, MEK6) activates both SAPK2a and SAPK2b (Cuenda et al., 1996; Han et al., 1996; Cuenda et al., 1997). SAPK2a (RK, p38, CSBP kinase, Mxi2) (Galcheva Gargova et al., 1994; Han et al., 1994; Lee et al., 1994; Rouse et al., 1994) and SAPK2b (p38β) (Jiang et al., 1996) phosphorylate and activate the transcription factors Elk-1 and CHOP (Price et al., 1996; Wang and Ron, 1996). In addition these SAPKs phosphorylate and activate the protein kinases MAP-KAP-K2 (MAPK activated protein kinase2) and MAP-KAP-K3 (Rouse et al., 1994; Clifton et al., 1996; McLaughlin et al., 1996), which in turn phosphorylate Hsp27 and the transcription factor CREB (Huot et al., 1995; Tan et al., 1996). The MKK3/SAPK2a pathway is activated in vivo by activated Cdc42 and Rac (Zhang et al., 1995) but not via MEKK1–4. Candidate regulators of this pathway are the p21 activated

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kinases. This group of kinases, PAK1 (hPAK65) and related isoforms (PAK2 and PAK3) are activated by binding RacGTP or Cdc42-GTP, and can stimulate SAPK1 and SAPK2 activation in vivo (Manser et al., 1994; Manser et al., 1995; Martin et al., 1995; Pombo et al., 1995; Zhang et al., 1995). Yeast genetic studies place Ste20, the homologue of hPAK1 immediately upstream of Stel 1, the homologue of MEKK1 (Herskowitz 1995), however, how PAK1 activates SAPK1 and SAPK2 is presently unclear. It does not directly activate MEKK1 and although its most likely targets are MKK4 and MKK3 or MKK6, PAK1 has not yet been shown to phosphorylate any of them (Fanger et al., 1997). Another candidate for upstream regulator of the SAPK2 (and SAPK1) pathway is ASK1, a kinase distantly related to MEKK1, that activates MKK4, MKK3 and MKK6 (Ichijo et al., 1997). Whether ASK1 is regulated by Rac and Cdc42 has not yet been determined, although preliminary data suggest it may be activated by TNFα (Ichijo et al., 1997). Also cAbl is required for SAPK1 and SAPK2 activation in response to some DNA damaging agents but not UV light (Pandey et al., 1996), but it is far from clear at what level this tyrosine kinase feeds into the SAPK activation module. Less well characterised SAPKs are SAPK3 (Erk6) and the recently cloned SAPK4, which are both activated by MKK6 and which both phosphorylate ATF2; no other substrates have yet been identified (Mertens et al., 1996; Cuenda et al., 1997; Goedert et al., 1997). Finally, Erk5 (Bmk1) is specifically activated by MEK5, but has no known physiological substrates (Abe et al., 1996). Clearly there are many SAPK substrates other than the few transcription factors and kinases known to date that need to be identified before the full biochemical consequences of activating SAPK cascades will be known. Role Of The MAPK And SAPK Cascades In Apoptotic Signalling Does the balance of MAPK vs SAPK activation determine cell fate? There is significant overlap in the stress stimuli that activate the SAPK cascades and those that can induce apoptosis. One of the first studies that implicated activation of the SAPKs and MAPKs in determining cell fate was that of Xia et al. (1995). They showed that 6h after the withdrawal of NGF from PC12 cells and prior to any of the characteristic signs of apoptosis, there is sustained activation of SAPK1 and SAPK2 and inhibition of MAPK. Expression of constitutively activated MEKK1, or coexpression of activated MKK3 and SAPK2 induces apoptosis of PC12 cells in the presence of NGF, while expression of dominant negative c-jun, dominant negative MKK4 or dominant negative MKK3 blocks apoptosis induced by NGF withdrawal or activated MEKK1. Conversely, activated MKK1 prevents apoptosis induced by NGF withdrawal (Xia et al., 1995). These results suggested that SAPK activation is pro-apoptotic while MAPK activation is anti-apoptotic and that the balance of activities in these two pathways determines cell fate. Two other studies lend support to this yin and yang hypothesis. First, in HL60 cells, ceramide, a second messenger induced by Fas ligand and TNFα induces activation of SAPK1 and apoptosis. This outcome is modified by sphingosine-1-phosphate which reduces SAPK activation and stimulates MAPK

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activation (Cuvillier et al., 1996). Second, TNFα robustly activates SAPK1, weakly activates MAPK and induces apoptosis in L929 cells. In contrast, FGF-2 activates MAPK and protects L929 cells against the apoptotic effect of TNFα without affecting TNFα-induced SAPK1 activation. The protective effect of FGF-2 is abrogated if MAPK activation is blocked with a MEK1 inhibitor (Gardner and Johnson, 1996). Interestingly, in this study it was also observed that oncogenic mutant Ras, which does not activate MAPK in L929 cells, also partially protects against TNFα-induced apoptosis; in hindsight this is probably explained by the activation of PI3K and Akt by Ras. Does MAPK activation promote cell survival? Other data also argue for an anti-apoptotic effect of activated MAPK. Thus, dominant negative MEK inhibits IL3-induced MAPK activation in BAF3 bone marrow derived cells and increases the level of IL3 required to prevent apoptosis. However, the expression of constitutively activated MEK does not prevent apoptosis when IL3 is withdrawn (Perkins et al., 1996). Leverrier et al. (1997) examined this system in more detail and concluded that MAPK activation is required for the induction of Bcl-X gene expression that correlates with the longterm survival of BAF3 cells returned to IL3 after a period of deprivation (Leverrier et al., 1997). In another model system, GM-CSF prevents apoptosis of haemopoietic cells that express wild type GM-CSF receptor but not cells that express a truncated GM-CSF receptor that does not activate Ras; this deficiency is complemented by coexpression of oncogenic mutant Ras, which restores MAPK activation (Kinoshita et al., 1995). Again these results might also be explained by the restoration of Ras dependent PI3K activation. Cardiotrophin-1, a potent survival factor for cardiac myocytes, provides another example; expression of dominant negative MEK1 or treatment of myocytes with the MEK inhibitor PD098059 both block the survival effect of cardiotrophin-1 (Sheng et al., 1997). Other groups have concluded that MAPK has no role in signalling for cell survival. The survival effect of IGF-1 on Rat-1 cells is not replicated by EGF or thrombin, which both robustly activate MAPK. And although overexpression of the EGFR in Rat-1 cells does render EGF anti-apoptotic against UV-B, this effect is still blocked by wortmannin (Kulik et al., 1997). These results argue against a significant role for MAP kinase in protecting against UV-B induced apoptosis. Raf targeted to the plasma membrane using the Ras localization sequences (RafCAAX), robustly activates MAPK (Leevers et al., 1994; Stokoe et al.,1994) but does not protect 32.D3 cells from apoptosis following withdrawal of IL3 (Wang et al., 1996). Conversely, Raf targeted to the mitochondrial membrane using the Bcl-2 localization sequences does not activate MAPK, but does protect 32.D3 cells from apoptosis following IL3 withdrawal, probably by stimulating the phosphorylation of BAD (Wang et al., 1996). This study reveals one of the few known links between signalling molecules and the apoptotic effector machinery, but it also demonstrates that MAPK activation cannot promote cell survival. Dudek et al. (1997) reached similar conclusions when they observed that growth factors such as BDNF, which strongly activate MAPK, but which do not activate PI3K, could not promote the in vitro survival of cerebellar neurons. Finally, there are studies showing that, persistent MAPK activation can actually enhance apoptosis. For example, the expression of RafCAAX or Ras mutants that selectively activate Raf enhances myc-induced apoptosis in fibroblasts (Kauffmann Zeh et al., 1997). Similarly

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bufalin induces MAPK activation and apoptosis in U937 cells but the effects of bufalin are abrogated if MAPK activation is blocked using an antisense cDNA against MEK1 (Watabe et al., 1996). Taken together these data are not generally supportive of a significant role for MAPK in signalling cell survival. The few examples where a positive effect of MAPK is seen may be peculiar to the cell system and apoptotic stimulus. In these cases it is noteworthy that sustained MAPK activation and nuclear translocation correlated with survival, suggesting that MAPK may have some role in regulating the commitment to apoptosis when new gene expression is required. Is SAPK activation required for apoptosis? From the evidence available there is clearly no consensus that MAPK activation has a consistent role in promoting cell survival. If we then exclude the concept that the balance of activity in the MAPK and SAPK pathways determines cell fate, what of the simpler hypothesis that SAPK activation is pro-apoptotic? The hypothesis is supported by two lines of evidence. First, activation of SAPK1 and SAPK2 precedes apoptosis in response to stimuli that include TNFα, UV and γ-irradiation, Fas ligand, chemotherapeutic drugs and other cellular stresses (Xia et al., 1995; Chen et al., 1996; Cuvillier et al., 1996; Heidenreich and Kummer, 1996; Kyriakis and Avruch, 1996; Verheij et al., 1996; Wihon et al., 1996; Chauhan et al., 1997). Secondly, expression of constitutively activated MEKKs or MKKs (SKKs) can induce apoptosis while dominant interfering MEKKs or MKKs (SKKs) can inhibit apoptosis, for example the data of Xia et al. (1995) in PC12 cells discussed earlier. Likewise, dominant negative SEK1(MKK4) or dominant negative c-jun protect U937 cells from apoptosis induced by ceramide, UV-C, heat shock, ionizing radiation and hydrogen peroxide(Verheij et al., 1996) and dominant negative SEK1(MKK4) can block apoptosis induced by heat shock and cis-platinum (Zanke et al., 1996). Moreover, ASK1, a novel SAPKKK that activates both SAPK1 and SAPK2, induces apoptosis when conditionally overexpressed in mink lung epithelial cells (Mv1Lu). Interestingly, endogenous ASK1 is activated when Mv1Lu cells are treated with TNFα and Mv1Lu cells are partially protected from TNFα-induced apoptosis by dominant negative ASK1 (Ichijo et al., 1997). These data are consistent with SAPK activation being required and sufficient for apoptosis. In contrast, constitutively activated MEKK1 induces apoptosis when microinjected into Swiss 3T3 and REF52 fibroblasts and enhances the apoptotic response of these cells to UV-C when conditionally expressed. But although MEKK1 activates SAPK1, dominant negative SAPKl does not suppress the apoptotic effect of activated MEKK1 in Swiss 3T3 and REF52 fibroblasts. Thus while MEKK1 activation is apoptotic, this effect appears not to be mediated via SAPK activation (Johnson et al., 1996), indicating that MEKK1 must have substrates other than SEK1 that are relevant for apoptotic signalling. The contribution of SAPK activation to TNFα and Fas-induced apoptosis is also debated. The issue is not simple because, as discussed elsewhere in this book, Fas and the TNFR, in addition to activating SAPKs, also activate the FADD/FLICE/caspase death pathway; Fas directly interacts with FADD while TNFR does so via the adapter protein, TRADD. It has recently been shown that Fas activates the SAPK pathway via a novel protein called Daxx,

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although how Daxx actually regulates SAPKKKs is unclear (Yang et al., 1997). Accumulating evidence suggests that the FADD pathway is the more important one for signalling Fas or TNFα-induced apoptosis. For example, Liu et al. (1996) show that while FADD overexpression is sufficient to induce apoptosis it does not activate either SAPK or NFκB, leading them to conclude that SAPK activation is not required for TNFα-induced apoptosis. Consistent with this conclusion, dominant negative FADD blocks TNFα-induced apoptosis in Hela cells but has no effect on TNFα-induced SAPK activation (Natoli et al., 1997). Similarly, crosslinking Fas induces SAPK activation and apoptosis in Jurkat cells, but whereas dominant negative SEK1(MKK4) effectively abrogates SAPK activation, it does not block apoptosis (Lenczowski et al., 1997). The observation that Daax overexpression robustly activates SAPK but does not induce apoptosis, is also consistent with there being no role for SAPK activation in Fas-induced apoptosis. However, dominant negative Daax abrogates Fas-induced apoptosis of HeLa and 293 cells, clearly invoking a contribution of SAPK activation to Fasinduced apoptosis (Yang et al., 1997). Caspases and SAPKs, who regulates whom? Are the caspases downstream targets of the SAPKs? In favour of this hypothesis, ICE inhibitors have been shown to block apoptosis induced by NGF withdrawal from PC12 cells, or etoposide treatment of U937 cells, but to have no effect on the coincident SAPK activation (Park et al., 1996; Seimiya et al., 1997). In direct contrast, other data place SAPK activation downstream of the caspases. For example, ICE inhibitors and crmA can block both Fas-induced apoptosis and Fas-induced SAPK activation in Jurkat cells as well as a variety of T and B cell lines (Cahill et al., 1996; Lenczowski et al., 1997). Juo et al. (1997) have extended these observations. They observe that Fas activation of SAPK2 correlates with the onset of apoptosis in Jurkat cells, and that the expression of constitutively activated MKK3 potentiates Fas-induced apoptosis. Pharmacological ICE inhibitors completely block Fasinduced activation of SAPK2 and Fas induced apoptosis, but interestingly do not block SAPK2 activation induced by osmotic shock. These results demonstrate that SAPK2 is activated by at least two mechanisms: a caspase dependent mechanism activated by Fas, and a caspase independent mechanism activated by osmotic shock (Juo et al., 1997). As with Fasinduced apoptosis, anoikis in epithelial cells is accompanied by activation of both ICE/LAP3 caspase and SAPK1. Expression of Bcl-2 or crmA protects against anoikis and also abrogates both caspase and SAPK activation (Frisch et al., 1996), which also places SAPK1 downstream of caspase activation. A mechanism for the control of SAPK activation by the caspases has recently been uncovered (Cardone et al., 1997). MEKK1 is activated when cells undergo anoikis, but MEKK1 activation induced by disruption of cell matrix results from a specific proteolytic event that generates an activated 78kDa C-terminal MEKK1 cleavage product. The cleavage event and MEKK1 activation are both blocked by caspase inhibitors that specifically target caspases which recognize DEVD motifs. Cleavage resistant MEKK1, constructed by mutating the caspase recognition sequence, inhibits caspase 7 activation and partially protects MDCK cells against anoikis. Taken together these data suggest that MEKK1/MKK3/SAPK and caspase 7 comprise a positive feed back loop, where activated MEKK1 activates caspase 7 via the SAPK pathway, and activated caspase 7 in turn activates more MEKK1 (Cardone et al., 1997). As discussed earlier, MEKK1 also has additional

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substrates, notably IκB: thus MEKK1 activation will result in coincident SAPK, caspase and NFκB activation. The precise outcome of these events will likely be influenced by cell context. Effect of cell context on the outcome of SAPK activation A striking example of the effect of cell context on the outcome of activation of the MEKK/ MKK4/SAPK1 cascade is that of lymphocytes. In B lymphocytes CD40 ligation activates SAPK1, but not MAPK, while cross-linking IgM activates MAPK. CD40 ligation rescues B cells from apoptosis induced by anti-IgM without affecting CD40 stimulated SAPK activation (Sakata et al., 1995). Similarly, B cell receptor activation in WEHI-231 cells strongly activates MAPK and weakly activates SAPK1, while CD40 crosslinking strongly activates SAPK1 and SAPK2. B cell receptor activation induces apoptosis of these cells which is blocked by activating CD40; thus apoptosis correlates with MAPK activation while cell survival correlates with coactivation of SAPK1, 2 and MAPK (Sutherland et al., 1996). Nishina et al., (1997) deleted SEK1(MKK4) in ES cells by homologous recombination. In SEK1−/−ES cells, SAPK1 is no longer activated by heat shock or the protein synthesis inhibitor anisomycin but is activated normally by UV irradiation and osmotic stress, while SAPK2 is activated normally in response to anisomycin and UV irradiation. Interestingly, T cells derived from SEK1−/−mice are no less sensitive to cell death induced by γ-irradiation, UV irradiation, heat shock, anisomycin or cisplatinum than SEK1+/+ mice, but are more sensitive to apoptosis induced by TCR activation. Thus a SEK1 regulated pathway protects T cells from TCRinduced apoptosis. Summary and speculation Some of the apparent discrepancies between the studies discussed here may be explained by the different end points used to score apoptosis, ranging from relative early events, such as morphological changes and DNA fragmentation assays to longer term cell survival assays. Given these differences it is not unreasonable to make the general conclusion that SAPK activation is involved in signalling for apoptosis. A specific model can be advanced, however, that embraces most of the data on SAPK activation discussed in this chapter. The model envisages activation of MEKK1 as the key event in triggering an apoptotic cascade. Since MEKK1 has IκB kinase activity and can phosphorylate and activate MKK4, it is tempting to speculate that it has other, as yet unidentified substrates that directly activate the apoptotic effector machinery and are essential for its apoptotic effect. The main role of SAPK activation then becomes one of participating in a positive feed back loop for activating caspases. Activated caspases further activate MEKK1 and so accentuate both SAPK activation and the activation of the unknown MEKK1 substrates. In this model SAPK activation is not essential for MEKK1 triggered apoptosis but facilitates the process by accelerating caspase activation. Inhibiting SAPK activation would, therefore, normally delay rather than prevent apoptosis. However, it is possible to envisage cell fate being altered by inhibiting SAPK activation if signalling in cell survival pathways is sufficiently great to balance the pro-apoptotic effects of MEKK1, weakened by loss of the SAPK/caspase feed back loop. Relevant survival signals would

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include Akt activity, and the anti-apoptotic effect of MEKK1-activated NFκB. Conversely, overexpression of constitutively activated MKK4/SAPK1 stimulates inappropriate caspase activation which cleaves MEKK1 and sets in train MEKK1 directed apoptosis. SAPK activation in one sense is a surrogate marker for MEKK1 activation whose substrate(s) other than SAPK may be the critical ones for inducing apoptosis. Caveats invoking cell and stimulus contexts must still be applied to this model, not least to account for the different role that SAPK activation plays in promoting survival of some lymphoid cells. CONCLUDING REMARKS Cell fate is determined by the prevailing balance of survival and pro-apoptotic signals. An overview of the studies discussed here leads to the following conclusions about the roles of PI3K/Akt, SAPKs and MAPKs in transducing these signals. A major signal transduction pathway that promotes cell survival comprises PI3K mediated activation of Akt. The upstream activation mechanisms of this pathway are well understood but nothing yet is known about the Akt targets that negatively regulate the apoptotic effector machinery. There is no consensus that MAPK activation has any significant role in promoting cell survival. It is probable that the few examples where MAPK does appear to be anti-apoptotic are peculiar to the cell system and apoptotic stimulus. Apoptosis is almost universally accompanied by SAPK activation. A strong case can be made for a major pro-apoptotic signalling pathway that involves MEKK1 activating as yet unknown substrates. Nevertheless, SAPKs also play a role in ensuring rapid and efficient cell suicide probably by participating in a positive feedback loop that activates caspases which in turn feed back onto MEKK1. The present challenge in this exciting area of biology is to extend these signal transduction pathways onto the direct regulators of apoptosis. The key lies in as yet unidentified subtrates of Akt/PKB and possibly also of MEKK1. ACKNOWLEDGEMENTS I would like to thank: Rob McPherson for compiling Figure 4.2, Kum Kum Khanna, Neal Walker, Glenda Gobe and Rob McPherson for critically evaluating the manuscript, and the Royal Children’s Hospital Foundation for continuing support. REFERENCES Abe, J.I., Kusuhara, M., Ulevitch, R.J., Berk, B.C. and Lee, J.D. (1996) Big mitogen activated protein kinase (BMK1) is a redox sensitive kinase. J. Biol. Chem., 271, 16586–16590. Adams, J.M., Houston, H., Allen, J., Lints, T. and Harvey, R. (1992) The hematopoietically expressed vav proto-oncogene shares homology with the dbl GDP-GTP exchange factor, the bcr gene and a yeast gene (CDC24) involved in cytoskeletal organization. Oncogene, 7, 611–8.

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5. PROTEIN KINASE C ISOENZYMES: EVIDENCE FOR SELECTIVITY IN THE REGULATION OF APOPTOSIS JANET M.LORD†, ELIZABETH M.DEACON, LORNA McMILLAN, GARETH GRIFFITHS, HEMA CHAHAL AND JUDIT PONGRACZ Department of Immunology, Birmingham University Medical School, Birmingham B152TT, UK Several disease model systems are now under active consideration for intervention at the level of the apoptotic programme. For example, the induction of apoptosis offers an alternative to cytostatic or differentiation therapies in the treatment of cancer. Many of the studies to identify those factors regulating apoptosis, that may be suitable for therapeutic intervention, have centred upon characterisation of the signal transduction pathways involved in the regulation of cell proliferation and apoptosis. One signalling element implicated in the regulation of cell proliferation and apoptosis, and already a recognised target for therapeutic modulation, is protein kinase C (PKC). PKC is a multigene family consisting of 11 isoenzymes which are regulated independently and are proposed to play specific roles in the regulation of cell functions, including apoptosis. We review here the experimental evidence that has been published concerning the involvement of PKC isoenzymes in the regulation of apoptosis. Whilst there is clearly still much contradictory data in this field of research, we propose that the weight of evidence suggests that the activation of PKC-alpha and beta constitutes a survival signal in many cell types. Whether the two splice forms of PKC-beta may have differential roles in the control of proliferation and apoptosis is also discussed. In contrast we have concluded that one PKC isoenzyme, PKCdelta, is activated during apoptosis and represents a pro-apoptotic PKC. The role of PKC and apoptosis in disease pathogenesis is also considered, with emphasis placed upon colorectal cancer.

† Corresponding Author: Tel: 44–121–414 4399. Fax: 44–121–414 3599. e-mail: Lord JM@novel!2. bham.ac.uk.

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PROTEIN KINASE C The realisation that apoptosis is a fundamental cellular process in development and morphogenesis (Kerr et al., 1974; Kerr et al., 1987; Kerr et al., 1972; Savill et al., 1993; Smith et al., 1989; Wyllie, 1980) and moreover, that its improper regulation impacts upon the initiation and progression of disease states(Savill et al., 1989; Williams, 1991), has led to the broadening of disease model systems under consideration for intervention at the level of the apoptotic programme. For example, it has become clear that several anti-cancer agents already in use are potent inducers of apoptosis (Dive and Hickman, 1991; Fisher, 1994). Many of the studies to identify those factors regulating apoptosis, have focussed upon identification of the signal transduction pathways involved in the regulation of cell proliferation, differentiation and apoptosis. More specifically, efforts have been concentrated on determining exactly how these pathways interact and how their disregulation leads to pathogenesis (Martin, 1982; Whitehouse et al., 1982; Grigg et al., 1991; Bursch et al., 1992; Groux et al., 1992; Kerr et al., 1994). One signalling element implicated in the regulation of cell proliferation, differentiation and apoptosis, and already a target for the design of novel therapeutic modalities, is protein kinase C (Gescher, 1992). Protein kinase C (PKC) is a serine/threonine kinase, which functions as a key element in signalling pathways regulating a variety of cell functions (Hug and Sarre, 1993; Nishizuka, 1992). PKC was first discovered in rat brain extracts by Nishizuka and co-workers (Inoue et al., 1977) who described it as a histone protein kinase that displayed a phospholipid and calcium dependency and in addition required diacylglycerol (DAG) for full enzyme activity. Interest in PKC as a signalling molecule increased very dramatically, following the discovery that this kinase was the cellular receptor for the tumour-promoting phorbol esters (Castagna et al., 1982). The range of cell functions known to be regulated through the PKC signalling pathway is extensive and includes, cell proliferation and differentiation (Clemens et al., 1992), secretion (Lord and Ashcroft, 1984), cytoskeleton function (Jaken et al., 1989; Owen et al., 1996), cell-cell contacts (Barry and Critchley, 1994), gene expression (Baudier et al., 1992; Li et al., 1992)and cell survival (Lotem et al.,1991; Pongrac et al., 1994; Lucas and Sanchez-Margalet, 1995). The pleiotropic involvement of PKC in the control of cellular activity raises the question of how specificity of biological action can be maintained, if a wide range of receptors are linked to the PKC signalling pathway. This paradox began to be resolved when the screening of rat cDNA libraries revealed the existence of several different, but closely related, isoenzymes of PKC (Parker et al., 1989). As most cells express several PKC isoenzymes and their amino acid sequences show a high degree of conservation across mammalian species (Dekker and Parker, 1994), it is likely that the individual PKC isoenzymes have specific rather than overlapping cell functions. PKC Isoenzymes The PKC isoenzyme family consists of multiple genes, defined by sequence homology and distinct sequence motifs (Figure 5.1). The 11 members identified to date, display differential properties with regard to tissue distribution. PKC-α, -δ and -ζ are the most widely distributed (Nishizuka, 1984; Wada et al., 1989; Wetsel et al., 1992) of the PKC

152 LORD, DEACON, MCMILLAN, GRIFFITHS, CHAHAL & PONGRACZ

Figure 5.1 Structure of the 3 main groups of the PKC isoenzyme family. (Key: c, classical; n, novel; a, atypical; V, variable region; C, constant region)

isoenzymes so far assessed. In contrast, PKC-γ is expressed exclusively in the central nervous system (Nishizuka, 1988), PKC-θ predominantly in skeletal muscle and haemopoietic cells (Osada et al., 1992) and PKC-η expression is greatest in skin and lung tissue, with only low levels detected in the brain and spleen (Osada et al., 1990). In addition, the isoenzymes of PKC differ in their susceptibility to regulation by co-factors, subcellular location, substrate specificity and down-regulation by proteolytic cleavage (reviewed by (Hug and Sarre, 1993)). Thus, although all of the isoenzymes transduce intracellular signals which involve the generation of lipid species, they appear to be regulated independently to effect an isoenzyme specific response (Nishizuka, 1992; Dekker and Parker, 1994). Detailed descriptions of the biochemistry and regulation of PKC have been covered in several recent reviews (Nishizuka, 1992; Hug and Sarre, 1993; Dekker and Parker, 1994; Goodnight et al., 1994; Newton, 1995), and will consequently require only a brief consideration here. Activation of PKC isoenzymes Whilst all PKC isoenzymes contain an amino terminal phosphatidylserine (PS)binding domain, the PKC isoenzyme family can be divided into three main groups, based on their additional activation requirements and ability to respond to phorbol esters (Figure 5.1). The latter bind to the regulatory domain of PKC, mimicking the acute physiological activation by DAG: – Classical, c-PKCs: -α , -β1(ll), -β2(I)* and -γ are calcium dependent and activated by phorbol esters; – Novel, n-PKCs: -δ, -ε, -η and -θ are calcium independent, but can be activated by phorbol esters; – Atypical, a-PKCs: -ζ, -λ/i and -µ are calcium independent and do not bind phorbol esters. Suggested physiological activators of these isoenzymes include ceramide (Lozano


et al., 1994; Muller et al., 1995) and phosphatidylinositol 3, 4, 5 trisphosphate* (Nakanishi et al., 1993).

DAG which is generated as a result of phospholipid hydrolysis will be able potentially to regulate both the c-PKCs and the n-PKCs. However, agonist stimulated phosphoinositide hydrolysis produces DAG and a calcium signal, via the generation of inositol 1, 4, 5trisphosphate (IP3). Both of these second messengers would be required for the full activation of the c-PKC isoenzymes. In contrast, the n-PKCs do not require the calcium signal and are able to respond to DAG produced in the absence of calcium. In support of this assertion, Ha and Exton have shown that signalling through thrombin and PDGF receptors results in the selective activation of n-PKCs, via DAG produced by the action of phospholipase D upon phosphatidylcholine (Ha and Exton, 1993). The PKC isoenzymes show another level of complexity of regulation in that they appear to be differentially responsive to lipid species. For example, activation of c-PKCs is further enhanced by cis-unsaturated fatty acids and lysophosphatidylcholine; free fatty acids activate PKC-ε and -ζ, but inhibit PKC-δ (Kikkawa et al., 1988; Lee and Bell, 1991; Hug and Sarre, 1993) and PKC-η is most effectively activated by cholesterol sulphate (Gschwendt et al., 1994). Ceramide is reported to activate PKC-ζ (Muller et al., 1995), but inhibits the activation of PKC-α (Lee et al., 1996). Thus the activation of specific PKC isoenzymes will vary dependent upon the lipid species generated as second messengers following receptor ligation. In addition, analysis of total cellular DAG in Swiss 3T3 fibroblasts, has revealed a complex mixture of 27 different molecular species, with different fatty acid compositions. Stimulation of these cells with Epidermal Growth Factor induced an increase in only a few DAG species (Pettit et al., 1994). Although differential activation of PKC isoenzymes by DAG species has not been reported as yet, this could clearly provide another route to selectivity in signalling through PKC. Our own preliminary studies have shown that stearoylarachidonyl glycerol production is associated with activation of PKC-β during the G2/M stage of cell cycle in U937 cells (E.M. Deacon, unpublished observations). Furthermore, the phospholipase D-derived DAG species generated in PAE cells, following treatment with LPA, did not activate PKC (Pettit et al., 1997). Further analyses are now required to establish the isoenzyme selectivity of lipid species generated in vivo and to confirm in vitro data, as the mode of presentation of lipid cofactors to PKC can affect their response (Palmer et al., 1995). Although a variety of lipids are able to modulate the activity of PKC isoenzymes selectively, it is unlikely that specificity of biological response is generated solely through differences in second messenger generation. It is clear that subcellular targeting following enzyme activation (Kiley et al., 1995) and substrate availability (Jaken, 1996), are key factors generating the functional specificity of the PKC isoenzymes.

* PKC-β isoenzymes are designated throughout in Arabic numerals according to the human cDNA nomenclature, with the Roman numeral designation for rat cDNA in parentheses, even if the original papers employed only one of the designations.


PKC translocation and subcellular targetting All PKC isoenzymes possess catalytic and regulatory domains separated by a hinge region, V3, (Figure 5.1) which is subject to proteolysis (Kishimoto et al., 1983). Dependent upon the class of PKC isoenzyme, the regulatory domain can include a calcium binding and a DAG/phorbol ester binding site. Activation of PKC isoenzymes requires their interaction with membrane phospholipid, anionic species such as PS being most effective (Kikkawa et al., 1988). In addition, inactive PKCs are located predominantly, though not exclusively, in the cytosol, though many of the known substrates for PKC are cytoskeletal or membrane proteins (Jaken, 1996). Thus activation of PKC is associated with its redistribution and association with intracellular membranes, to allow interaction with PS and substrates. In vitro studies have shown that recombinant PKC isoenzymes show very little substrate preference (Fujise et al., 1994) and a degree of pre-positioning of PKC isoenzymes may consequently be required to achieve the selective phosphorylation of substrates reported for in vivo studies. Differential subcellular localisation of PKC isoenzymes has been reported in a variety of cell types. PKC-α is localised to focal contacts in normal REF52 cells (Liao et al., 1994b), PKC-δ is associated with vimentin intermediate filaments in differentiated HL60 cells (Owen et al., 1996) and PKC-β2(l) is localised to the microtubule cytoskeleton in proliferating U937 cells (Kiley and Parker, 1995). Clearly the location of substrates such as talin, vimentin and microtubule associated proteins will contribute to the targeting of PKC isoenzymes following their activation. This proposal is supported by data from a variety of studies reporting that the direction of translocation of PKC isoenzymes is highly variable. For example, in Swiss 3T3 cells treated with bombesin, PKC-α translocates to the cell plasma membrane. In the same cells treated with IGF-1, PKC-α is associated with the nuclear membrane (Divecha et al., 1991). PKC-β translocates to the nucleus when K562 cells are treated with bryostatin-1 and to the plasma membrane following treatment with phorbol dibutyrate (Hocevar and Fields, 1991). Clearly such differences will influence the actions of PKC isoenzymes, most notably regarding substrate availability (Kiley et al., 1995) and will dictate the role of PKC isoenzymes in cell function. Exactly how targeting of PKCs before and after activation are related and effected at the molecular level is not clear. Several laboratories have used approaches such as interaction cloning and yeast-two hybrid screening to identify proteins that may localise PKCs to particular regions of the cell. These studies have identified a variety of proteins, including known PKC substrates and novel PKC-interacting proteins (Chapline et al., 1993; Staudinger et al., 1995), that shed some light on this problem. For example, several PKC binding proteins are not substrate proteins and appear to act to localise PKC following its activation. Such proteins include the receptors for activated C kinase (RACKs) first described by Mochly-Rosen and coworkers (Ron et al., 1994) and the pleckstrin homology domain of Brutons tyrosine kinase, which binds PKC-µ (Sidorenko et al., 1996). Furthermore, Dong et al., showed that one of the PKC binding proteins, 35H, was phosphorylated following agoniststimulated second messenger generation and that the phosphorylated protein had a greatly reduced affinity for PKC (Dong et al., 1995). These observations have led Jaken to propose that the relocation of PKC isoenzymes following activation (Kiley and Parker, 1995), may be a consequence of release of inactive PKCs from binding proteins and their


diacylglycerol-induced association with substrates in the membrane/cytoskeleton (Jaken, 1996). If more of these PKC binding proteins are identified and shown to be located at specific sites within the cell, they may provide the selective localisation that would allow discrete activation of PKCs and interactions with specific substrates. Recently, proteins have been identified which effect the compartmentalisation of cAMP-dependent protein kinase (PKA). The A-kinase anchoring proteins (AKAP) are a diverse group of proteins which bind the regulatory subunit of type 11 PKA isoforms and are located at distinct sites within cells, including the plasma membrane (Gray et al., 1997), sarcoplasmic reticulum (McCartney et al., 1995), nuclear matrix (Zhang et al., 1996) and actin cytoskeleton (Dransfield et al., 1997). Moreover, the AKAP proteins have been shown to bind protein phosphatase and PKC at a site distinct from PKA (Nauert et al., 1997) and may therefore also act as scaffolding proteins for PKCs and other signalling elements. Whether the different AKAP proteins show selectivity in their binding of PKC isoenzymes has not yet been reported. The regulatory domain of PKC appears to play a crucial role in targeting of inactive PKC and substrate interactions (Pears et al., 1991). For example, removal of the pseudosubstrate domain decreases its affinity for PKC binding proteins (Liao et al., 1994a). The regulatory domain of PKC contains several regions that are able to bind PS and diacylglycerol contributes to the translocation of PKC by increasing its affinity for PS (Newton, 1995). PKC substrates also contain PS binding domains (Chapline et al., 1993; Hyatt et al., 1994) and substrate proteins, such as the MARCKS protein, which are able to localise PS into high density regions. Such domains would allow PKC binding to PS and to other proteins, in effect PS may act as a bridge between PKC and its substrates in a ternary complex located at cell membranes (Jaken, 1996). Thus, substrate localisation is not the only factor affecting PKC translocation, the PS binding domains are important in enzyme activation (see above) and protein interactions. For example, the regulatory domain of PKC-α masks nuclear localisation sequences in inactive PKC (Liao et al., 1994a) and the cysteine rich motif zinc finger domain of PKC-ε mediates its localisation to the Golgi (Lehel et al., 1995). In summary, a model may be proposed in which differential generation of second messengers dictates which PKCs are activated in response to receptor ligation and the subcellular targeting of PKC isoenzymes pre- and post-activation, mediated by the PS binding domains on PKCs and PKC-binding proteins, will determine isoenzyme function in vivo. Interestingly, this model may already require slight modification to resolve the most recent data in the literature. Sawai and co-workers (Sawai et al., 1997) have reported that PKC-δ and ε were translocated from the membrane to the cytosol in response to the generation of ceramide, following treatment of cells with TNF-α and prior to the subsequent induction of apoptosis. However, rather than concluding that these isoenzymes were inhibited by ceramide, the authors suggest that the translocation to the cytosol leads to their proteolytic activation by cytosol based ICE proteases (see below). In this case, a second messenger has induced translocation of PKC and has possibly effected enzyme activation, but with translocation directed away from cell membranes. Whether the proteolyticallyactivated PKC isoenzymes in these studies ultimately translocated towards the membrane/cytoskeleton was not determined.


Proteolysis and Downregulation of PKC The regulatory domain of PKC also includes a motif resembling the consensus site for PKC substrates, xRxxS/TxRx, in which a serine/threonine residue is changed to alanine. This motif represents a pseudosubstrate site which blocks the catalytic site in inactive PKCs. Binding of an activator such as DAG, induces a conformational change removing any inhibition by the pseudosubstrate site and also renders the hinge region open to proteolytic cleavage (Newton, 1995). Proteolysis generates two distinct fragments, the regulatory domain and a catalytically active protein kinase domain known as PKM (Huang and Huang, 1986; Schaap et al., 1990). In the case of the c-PKCs, the hinge region contains cleavage sites for the calciumdependent neutral proteases calpains I and II (Kishimoto et al., 1989). Schaap et al. (Schaap et al., 1990) have also reported a cleavage site for trypsin in PKC-ε. Persistent activation of PKC, resulting from treatment with phorbol esters or certain growth factors (Olivier and Parker, 1994), leads to the downregulation of PKC protein. In most cases it is not known whether the sustained activation of PKC, or its subsequent downregulation, is important for the elicitation of cellular responses. Interestingly the PKC isoenzymes vary in their susceptibility to such cleavage. PKC-α has been shown to be relatively resistant to proteolysis mediated downregulation, whereas PKC-β and PKC-γ are more easily inactivated (Kochs et al., 1993). As will be discussed later, it has been shown recently that a n-PKC, PKC-δ, can be cleaved by the ICE-like protease CPP32 to generate a catalytically active 40kDa PKC (Emoto et al., 1996). This property was reported to be unique to PKC-δ, though a recent publication has claimed that PKC-ε is also susceptible (Sawai et al., 1997). Whether PKC isoenzymes are targets for other proteases remains to be established, but clearly this mode of activation could involve PKC in an even wider range of signalling pathways. The preceding information illustrates the complexity of PKC regulation and the factors that must be taken in to account when considering its role in apoptosis and disease. PKC involvement will be affected by the isoenzyme content of the tissue being studied, the second messengers generated following receptor ligation, degree of downregulation, direction of translocation and substrate availability. To date the majority of studies that have considered the role of PKC in apoptosis have not taken these factors into account. However, it is becoming clear that PKC does regulate apoptosis and that specific isoenzymes are involved in signals that promote or delay apoptosis. The following is an attempt to review the current literature in this area, with emphasis placed upon studies that have considered the role of individual PKC isoenzymes. PKC ISOENZYMES AND APOPTOSIS Apoptosis is a multi-step process, which can be broken down to four distinct stages: – signal generation and interpretation. Signals can be either intrinsic or extrinsic to the cell and will dictate whether a particular cell proliferates, differentiates or dies; – the commitment phase. This stage is variable in length and commences once the proapoptotic signal is received;


Table 5.1 Effects of TPA on Apoptosis.

– execution of the apoptotic programme. This stage includes all the characteristic morphological changes of apoptosis and is complete within 15–60 minutes (reviewed in (Earnshaw, 1995)); – recognition and removal of apoptotic cells. Changes that occur at the cell membrane during apoptosis allow recognition of dying cells by professional phagocytes or neighbouring cells (Savill et al., 1993). It is therefore possible that individual PKC isoenzymes may play different roles in regulating cell death and survival. Evidence supporting a role for PKC in the regulation of apoptosis is substantial, though contradictory (Table 5.1). Data concerning specific PKC isoenzymes are less abundant, but may provide the greatest insight into the involvement of PKC in cell survival processes. Studies to determine the role of PKC isoenzymes in cell survival have involved three broad approaches: (a) the use of pharmacological modulators of PKC activity with varying degrees of isoenzyme selectivity; (b) correlative approaches, assessing PKC isoenzyme expression and/or activation status with cell survival; (c) assessment of phenotypic changes in cells overexpressing wild type PKC isoenzymes, constitutively active or kinase defective PKC mutants. Rather than attempt to review the findings with regard to these distinct methodological approaches or with respect to specific cell types, this article will consider the data in relation to the role of the three functional classes of PKC isoenzymes. This approach should facilitate conclusions regarding individual isoenzymes and highlight those PKCs which appear to play a consistent role in apoptosis, rather than a cell specific involvement in cell survival. In addition, we will consider briefly the known substrates for the relevant PKC isoenzymes, in an attempt to identify potential down-stream targets that may mediate the effects of PKC upon cell survival. The chapter will then close with a discussion of the role of apoptosis in disease pathogenesis, concentrating upon examples in which alterations to PKC isoenzymes have also been documented.


c-PKC Isoenzymes This class of PKC isoenzymes are responsive to phorbol esters, calcium and a range of lipid co-factors, including diacylglycerols. The c-PKCs might therefore be expected to mediate the effects of ligands that induce apoptosis via generation of intracellular calcium and/or lipid hydrolysis products, or to be the target for phorbol ester mediated changes in cell survival. There is now an increasing body of data suggesting that PKC-α may be classed as an anti-apoptotic PKC, the picture concerning PKC-β is less clear and there is little data concerning the role of PKC-γ. As the latter is predominantly expressed in neuronal tissue, its function may be very restricted and relate to brain function. PKC-alpha PKC-α appears to be expressed universally and has been implicated in mitogenesis by several authors. For example, PKC-α was shown to translocate to the nucleus following treatment of llC9 fibroblasts with α-thrombin (Leach et al., 1992) and more recently, PKCα was shown to interact with Raf-1, leading to activation of the MAP kinase cascade and proliferation of NIH 3T3 cells (Li et al., 1995). Whilst the regulation of proliferation and apoptosis appear to be closely integrated (Evan et al., 1992), they are ultimately mutually exclusive events. By inference PKC-α activation would be expected to constitute a survival signal. Grant and co-workers showed that pre-treatment of HL60 cells with bryostatin 1 for 24 hours produced a significant down-regulation of c-PKC isoenzymes and potentiated Ara-Cinduced apoptosis. Mezerein, another potent PKC activator, did not produce the same degree of down-regulation and induced differentiation of HL60 cells, with no potentiation of Ara-C-induced apoptosis (Grant et al., 1996). These studies did not consider individual PKC isoenzymes and can only infer a protective role for c-PKCs as they did not assess changes in the n-PKC isoenzymes. However, studies in endothelial cells have suggested that PKC-α is involved specifically in mediating the protective effects of b-FGF against radiation-induced apoptosis (HaimovitzFriedman et al., 1994). The protective effects of bFGF were mimicked by short term treatment of cells with TPA and bFGF was shown to translocate PKC-α within 30 seconds of treatment. Moreover, depletion of PKC-α abrogated the effects of bFGF. More recently, modulation of PKC-α has been shown to be involved in the induction of apoptosis by ceramide. Ceramide is a lipid second messenger produced from the hydrolysis of sphingomyelin, following the activation of sphingomyelinase by extracellular ligands such as TNF-α and IFN-γ (Dressler et al., 1992). Ceramide mediates the effects of these extracellular ligands on cell differentiation, growth arrest and apoptosis. Two groups have shown that ceramide inhibits PKC-α, though they disagree on the molecular basis of this inhibition. Jones and Murray, working with mouse epidermal (HEL37) and human skin fibroblast (SF3155) cells, showed that ceramide induced apoptosis and inhibited the translocation of PKC-α (Jones and Murray, 1995). The effect appeared to be specific to PKC-α , with no effect on PKC-ε. In contrast, Lee and co-workers showed that ceramide did not inhibit translocation of PKC-α in MOLT-4 cells and could not inhibit PKC directly


in vitro (Lee et al., 1996). Inhibition of PKC enzyme activity by ceramide was in fact achieved by dephosphorylation of PKC-α (Lee et al., 1996). Phosphorylation is an important mode of regulation of PKC, which is first synthesised as a dephosphorylated, inactive precursor (Borner et al., 1989). Several groups have shown that phosphorylation of PKC within the activation loop leads to activation of the enzyme and dephosphorylation within this domain inhibits enzyme activity (Cazoubon et al., 1994; Orr and Newton, 1994). One of the cellular targets of ceramide is a ceramide-activated protein phosphatase, CAPP (Dobrowsky and Hannun, 1992). CAPP shares several properties with protein phosphatase 2A (PP2A). Interestingly, PP2A was able to dephosphorylate PKC (Dutil et al., 1994) and okadaic acid was able to reverse the actions of ceramide on PKC-α (Lee et al., 1996). These data suggest that PKC-α is a down-stream target in the ceramide survival regulatory pathway and that its inhibition allows for growth inhibition and the induction of apoptosis. As with most of the literature concerning PKC and apoptosis, the data concerning PKC-α does not allow for a uniform interpretation to be made. For example, induction of apoptosis by TPA in androgen-sensitive human prostate cells (LnCap) was accompanied by a 12 fold increase in PKC-α mRNA (Powell et al., 1996). TPA resistant LnCap cells had reduced levels of PKC-α and raised levels of PKC-µ (Powell et al., 1996). In addition, Devente et al., showed that MCF-7 cells over-expressing PKC-α were induced to apoptose when treated with TPA. This is in contrast to wild type MCF-7 cells which differentiated in response to TPA (Devente et al., 1995). However, the majority of the current literature suggests that PKC-α is involved in mitogenic signalling pathways in many cells. Indeed, PKC-α expression and/or activation status is raised in several cancers (Lord and Pongracz, 1995). Reduction in PKC-α expression, achieved by the use of anti-sense oligonucleotides, results in decreased proliferation in rat coronary vascular smooth muscle cells (Leszczynski et al., 1996). Encouragingly, antisense to PKC-α has been used very recently as a therapeutic approach to the treatment of glioblastoma. Reduced levels of PKC-α led to a significant reduction in tumour mass. Whether the reduced tumour growth was due to alterations in proliferation or apoptosis was not determined in these studies (Yazaki et al., 1996). PKC-beta PKC-β, as stated earlier, exists as two alternate splice forms β1(ll) and β2(l). The two splice forms differ by only approximately 50 amino acids in the C terminal regulatory domain, but this would appear to be sufficient to influence their distribution within the cell. For example, studies in U937 cells have shown that PKC-β1(ll) was present in the cytoplasm of proliferating cells and was specifically localised to vesicles containing β2-integrin molecules. In contrast, PKC-β2(l) was associated with the microtubule network, via binding to microtubule-associated proteins (Kiley and Parker, 1995). As targeting of PKC isoenzymes to specific locations within the cell may be a factor in determining isoenzyme specific function in vivo (Kiley et al., 1995), PKC-β1 and β2 may be differentially involved in the processes of cell differentiation, proliferation and apoptosis. Unfortunately, only very few studies have assessed the role of the individual PKC-β splice forms simultaneously and it is


therefore too early to be able to state whether or not they have differential roles in the regulation of cell survival. What is clear from several sources is that PKC-β plays a major role in the co-ordination of cell proliferation, differentiation and apoptosis. Studies of PKC isoenzyme expression in non-apoptotic and apoptotic cells showed that PKC-β expression was increased in apoptotic U937 cells (Pongracz et al., 1995b). PKC isoenzyme expression was also assessed with relation to the expression of Bcl-2 in human tonsil, these data revealed a negative correlation between expression of PKC-β and Bcl-2 (Knox et al., 1993). In addition, HL60 cells which were unable to differentiate or apoptose in response to phorbol esters, were shown to lack expression of PKC-β. Induction of PKC-β expression following treatment of cells with 1, 25-dihydroxyvitamin D3, restored responsiveness to phorbol esters (Macfarlane and Manzel, 1994). HL60 cells stop proliferating in response to phorbol esters such as TPA. Growth arrest is rapid (complete within 12h) and precedes acquisition of the differentiated monocyte phenotype and apoptosis. Precisely which stage of the differentiation/apoptosis process required PKC-β was not determined in these studies. It will be important to consider both PKC-βl and PKC-β2 in studies such as these, if we are to determine their precise role in cell proliferation and survival. A series of detailed studies from Fields and co-workers, primarily using HL60 cells, have established a role for PKC-β1(ll) in cell proliferation (see also PKC substrates section). They have shown that PKC-β1(ll) is a mitotic lamin kinase in haemopoietic cells (Goss et al., 1994). Analysis of enzyme translocation through the cell cycle revealed that this isoenzyme was activated and translocated to the nucleus at G2/M. The nuclear lamin proteins form a polymeric network which is regulated by multisite phosphorylation. Whilst p34 kinase is able to phosphorylate nuclear lamin proteins in vitro to bring about dissolution of the lamina complex (Peter et al., 1990), in vivo PKC-β1(ll) also phosphorylates lamin B in G2/M at a site distinct from p34cdc2 (Goss et al., 1994). The most recent report from this group has shown that PKC-β1(ll) was necessary for progression of cells through G2/M. Furthermore, lamin phosphorylation sites were analysed and site-directed mutagenesis employed to show that the sites phosphorylated by PKC were required to maintain lamin B protein in the cytoplasm during mitosis (Thompson and Fields, 1996). Thus, PKC-β1(ll) would appear to be primarily involved in the regulation of pro liferation, at least in haemopoietic cells. This proposal is supported by data from studies investigating the molecular basis of the suppression of apoptosis by the oncogene v-abl. Transfection of a temperature sensitive v-abl construct in to an IL3dependent haemopoietic cell line, abrogated apoptosis induced following growth factor withdrawal when cells were kept at the permissive temperature and v-Abl tyrosine kinase was expressed and activated (Evans et al., 1993). Prevention of apoptosis was later shown to be associated with the activation and translocation of PKCβ1(ll) towards the nucleus (Evans et al., 1995). Pretreatment of cells with calphostin C prevented nuclear translocation of PKC-β1(ll) and inhibited the suppression of apoptosis by v-Abl. Thus, like PKC-β, PKC-β1(ll) would appear to predominantly regulate cell proliferation and promote cell survival. Whether the inappropriate activation of PKC-β1(ll) in proliferating cells could in certain instances lead to growth arrest and apoptosis is one possibility that could be considered. As this isoenzyme is normally activated at G2/M, activation at any other stage of the cell cycle could induce a


form of “mitotic catastrophe”, postulated by several authors to result in apoptosis in cycling cells (Shi et al., 1994). With regard to PKC-β2(l) and apoptosis, the data are equivocal. Treatment of U937 cells with the the phorbol ester, TPA, induced growth arrest and differentiation towards monocytes. Treatment of cells with the deoxyphorbol ester, Doppa, which is selective for the activation of PKC-β in vitro (Ryves et al., 1991), did not induce differentiation and increased apoptosis (Pongracz et al., 1996). PKC-β2(l), the dominant splice form in these cells, was selectively translocated at concentrations of Doppa at or below 50nM. However, this effect was transient and after 1 hour other isoenzymes were also translocated. Furthermore, other authors have reported that Doppa is not PKC-β selective in vivo (Roivainen and Messing, 1993) and these data must therefore be interpreted with caution. We have found more recently that upregulation of PKC-β occurred in apoptotic human neutrophils (Pongracz et al., unpublished observations). These cells are fully differentiated and therefore do not proliferate and die spontaneously by apoptosis within 24–48 hours of leaving the bone marrow. Interestingly, this change in expression was restricted to the β2(l) isoform, with levels of PKC-β1(ll) remaining unchanged. Recent studies in small lung carcinoma cells (NC1 H209) also suggest a role for PKC-β and suggest that the effects of c-myc upon cell proliferation may be mediated via this isoenzyme (Barr et al., 1997). However, the data from these studies suggest a role for PKC-β2(l), rather than PKC-β1(ll), in the regulation of proliferation and apoptosis. PKC-β1(ll) and PKC-β2(l) were transfected into cells individually, either before or after transfection with c-myc. Overexpression of c-myc alone produced a significant increase in cell proliferation which was not affected by co-expression of PKC-β1(ll). In contrast, transfection of PKC-β2(l) alone extended cell doubling time and increased the fraction of cells in G0/G1. Co-expression of c-myc with PKC-β2(l) improved survival of cells in low serum, with cells in G0/G1 being particularly affected. In conclusion, PKC-β1 and β2 have both been implicated in the regulation of apoptosis, but the role of each splice form may vary with cell type and the compliment of isoenzymes expressed. At this stage, the few individual studies that have considered both beta isoforms, do appear to suggest their differential involvement in the regulation of apoptosis within a particular cell type. At this stage no further generalisations can be made as to their individual roles in the regulation of apoptosis. n-PKC Isoenzymes It is unlikely that apoptosis is regulated only by the classical PKC isoenzymes. Apoptosis is not associated with changes in intracellular calcium in all cells (Beaver and Waring, 1994) and although PKC-α is expressed in a very wide range of cells, many cells do not express PKC-β or PKC-γ. We have used a cell free approach to identify PKC isoenzymes that are required for apoptosis in human neutrophils. Nuclei isolated from healthy, non-apoptotic neutrophils were combined with the cytosol of neutrophils at an early stage of apoptosis. DNA fragmentation occurred within 30 minutes and was inhibited if PKC-δ was removed from the cytosol by immunoprecipitation (Pongracz et al., unpublished observations). Therefore, PKC-δ is implicated in the apoptotic process in neutrophils and studies from other laboratories are now also suggesting a principal role for this isoenzyme in the promotion of apoptosis.


PKC-delta PKC-δ, like alpha, appears to be expressed in almost every cell tested (Hug and Sarre, 1993). Several lines of evidence suggest that activation of PKC-δ may inhibit cell growth or cell cycle progression and induce apoptosis. Overexpression of PKC-δ in Chinese hamster ovary cells and NIH 3T3 cells induced growth arrest in G2/M (Watanabe et al., 1992). The marine compound Bistratene A also induced growth arrest in G2/M in HL60 cells (Griffiths et al., 1996), which was followed by apoptosis (Lord et al., 1995). Bistratene A also induced apoptosis in a Burkitts lymphoma cell line BM13674 (Song et al., 1992). We have shown recently that Bistratene A activated PKC-δ specifically, using both in vitro and whole cell PKC assay methods (Griffiths et al., 1996). These data suggest that activation of PKC-δ will cause cells to accumulate in G2/M and will eventually lead to apoptosis in haemopoietic cells. The argument in favour of a role for PKC-δ in the induction of apoptosis has been supported most clearly by a series of experiments reported by Kufe and co-workers. Emoto et al. (Emoto et al., 1995) originally purified and characterised a 40kDa protein kinase activity that was induced by a variety of agents which caused apoptosis in U937 cells, including Ara-C, TNF-α, Fas-ligation and UV-irradiation (Datta et al., 1996). Sequencing of the purified kinase revealed it to be a cleaved form of PKC-δ, containing the catalytic domain. Further analysis showed that full length PKC-δ was cleaved at DMQD330N during apoptosis, to produce a catalytically active PKC. The finding that PKC-δ was cleaved at a site adjacent to aspartic acid suggested the potential involvement of aspartate-specific cysteine proteases (Caspases), which are known to be activated during apoptosis. CPP32 is one of the caspases known to be involved in the execution of the apoptotic programme, its substrates include the DNA repair enzyme polyADP-ribose polymerase. Ghayur et al. (Ghayur et al., 1996) showed that a CPP32-like protease activity was responsible for the proteolytic activation of PKC-δ. Furthermore, transfection of U937 cells with the proteasegenerated fragment of PKC-δ was sufficient to induce apoptosis. Full length PKC-δ, or a kinase defective PKC-δ fragment, did not induce apoptosis. Finally, inhibition of apoptosis by overexpression of Bcl-2 or Bcl-XL was also associated with abrogation of PKC-δ cleavage (Emoto et al., 1995). Data from another group has suggested recently that the effects of Fasligation and TNF-α on apoptosis and PKC-δ cleavage may be mediated by ceramide (Sawai et al., 1997). Treatment of U937 and HPB-ALL cells with anti-Fas antibody, TNF-α or cell permeable ceramide analogues, raised cellular levels of ceramide and induced apoptosis. These agents also induced translocation of PKC-δ and ε from the membrane to the cytosol. As caspases are located within the cytosol, this translocation may well effect the proteolytic activation of PKC-δ and ε Although these authors did not present the relevant data, in the discussion section of this paper they report that translocation of PKC-δ and PKC-ε was accompanied by their proteolytic cleavage (Sawai et al., 1997). This latter observation is however in disagreement with published data, which have shown that the proteolytic activation of PKC in cells undergoing apoptosis was restricted to PKC-δ and did not include PKC-α, PKC-ε or PKC-ζ (Emoto et al., 1996; Emoto et al., 1995). Whilst the majority of available data suggest a pro-apoptotic role for PKC-δ, there are some exceptions in the literature. Leszczynski has used a PKC-δ antisense oligonucleotide to reduce expression of this isoenzyme selectively in rat vascular smooth muscle cells


(Leszczynski et al., 1995). Loss of PKC-δ in these studies resulted in increased apoptosis, leading to the suggestion that down-regulation, rather than activation, of PKC-δ was a signal for apoptosis. These studies need to be repeated in a variety of cell types before any firm conclusions can be made, but at the present time this proposal would appear to be at odds with the rest of the literature, which suggests a positive role for PKC-δ in the promotion of apoptosis. PKC-epsilon PKC-ε is the only PKC isoenzyme that has been shown to have full oncogenic potential, its overexpression in NIH 3T3 cells produces a transformed phenotype (Mischak et al., 1993). Despite this observation, the literature on the role of PKC-ε in the regulation of apoptosis is relatively sparse. Recent preliminary data have suggested that PKC-ε may function as an “early response” protein involved in entry of quiescent cells into cell cycle and DNA replication (Watson et al., 1996). This proposal is supported by the data of Li et al. (Li et al., 1996), which shows that PKC-ε is required for induction of c-myc and DNA replication in erythroleukaemia cells by erythropoietin. Mihalik et al. (Mihalik et al., 1996) have also shown that low concentrations of TPA (0.5ng/ml) produced growth arrest of HT58 human lymphoblastic cells in G1, which was correlated with the downregulation of PKC-ε. In addition, PKC-ε is expressed at high levels in tumorigenic rat colon epithelial cells (Perletti et al., 1996), neoplastic rat prostate cancer cells (Hrzenjak and Shain, 1995) and MCF-7 breast cancer cells (Manni et al., 1996). From the above data it is tempting to conclude that PKC-ε is a mitogenic isoenzyme, with a resulting inhibitory effect on apoptosis. However, an equal number of studies have produced evidence to the contrary and suggest that PKC-ε is proapoptotic in function. Kiss and Anderson (Kiss and Anderson, 1994) showed that carcinogens reduced the expression of PKC-ε in mouse embryo fibroblasts. The same group reported that the inhibition of apoptosis in prostatic carcinoma cells could be achieved by the activation and downregulation of PKC-ε (Rusnak and Lazo, 1996). Our own studies have shown that PKC-ε was significantly decreased in human colon cancer tissue (Pongracz et al., 1995a). Finally, glucocorticoidinduced apoptosis in immature thymocytes is dependent upon PKC. Iwata et al., have shown that PKC-ε was translocated from the cytosol to the particulate fraction in immature thymocytes in response to glucocorticoids, PKC-α and β were unaffected (Iwata et al., 1994). If PKC-ε is indeed an early response protein, then the outcome of activation of this isoenzyme may well depend upon the genes that are subsequently transcribed. a-PKC Isoenzymes The atypical PKCs have only recently been implicated in the regulation of proliferation and apoptosis. It is likely that our incomplete understanding of the factors regulating these isoenzymes has restricted the study of their role in cell regulation. However, one signalling intermediate that was identified as an activator of PKC-ζ in vitro was ceramide (Muller et al., 1995). Bearing in mind the weight of evidence suggesting a major role for ceramide in


mediating signals for cell death inducing ligands, further investigation of the role of PKC-ζ and other a-PKCs is warranted. PKC-zeta Although PKC-ζ was shown to be activated by ceramide in in vitro studies, evidence from several sources suggest that this isoenzyme is in fact involved in mitogenesis and is generally anti-apoptotic. Berra et al. have demonstrated that PKC-ζ interacts with key elements of the mitogenic signalling cascade downstream of Ras. Transfection of Cos cells with a constitutively active mutant of PKC-ζ led to the activation of MAP kinase and MEK and a kinase-defective dominant negative mutant of PKC-ζ impaired the activation of these kinases by serum and TNF-α (Berra et al., 1995). Moreover, the stimulation of stress-activated protein kinase (SAPK) was shown to be independent of PKC-ζ in these studies, giving further support to the proposal that this isoenzyme is not involved in signalling pathways leading to apoptosis. Whilst PKC-ζ does not appear to be involved in the pro-apoptotic pathway, it is apparent that to initiate apoptosis, it may be necessary to inhibit signalling through PKC-ζ. The expression of PKC-ζ is markedly reduced in apoptotic U937 cells (Pongracz et al., 1995b) and loss of this isoenzyme in rat vascular smooth muscle cells, following treatment with an anti-sense oligonucleotide, resulted in apoptosis (Leszczynski et al., 1995). More recently, Diazmeco et al., have shown that the product of the PAR-4 gene, which is induced during apoptosis in a variety of cells, interacts specifically with a-PKCs (Diazmeco et al., 1996). Interaction of PAR-4 with the regulatory domains of a-PKCs, including PKC-ζ, dramatically inhibited their enzymatic activity. Therefore, in contrast to the data regarding the c-PKCs and n-PKCs, the studies that have considered the a-PKCs appear to unanimously sug gest a proliferative and survival role for this group of isoenzymes. PKC Substrates In determining a role for PKC in the regulation of a specific cell function, such as apoptosis, it is clearly important to establish whether a particular biological effect is correlated with activation or downregulation of a specific isoenzyme. In addition, significant progress will only be made if we can define precisely how PKC isoenzymes mediate their effects on cell regulation. In this respect it is the identification of PKC substrates in whole cell studies that will increase our understanding of PKC in the future. PKC is known to phosphorylate a number of different substrates in vitro, which include other proteins involved in signal transduction (Berra et al., 1995), proteins regulating DNA synthesis (Baudier et al., 1992; Li et al., 1992), DNA modifying enzymes (Sahyoun et al., 1986; Bauer et al., 1992) and proteins involved in cell cycle control (Goss et al., 1994). The modification of any of these substrates could be expected to be involved in the execution of the apoptotic programme. For example, cell cycle arrest at the G1/S boundary induced by TNF-α, has been shown to involve dephosphorylation of Rb and to require the inactivation of PKC-α, possibly mediated by ceramide activated protein phosphatase (Lee et al., 1996). In addition, it is clear that several of the PKC substrates that have been identified in whole cell assays are cytoskeletal elements (Jaken et al., 1989; Murti et al., 1992; Owen et al., 1996). As the disassembly of


the cells cytoskeletal structure is a feature of apoptosis, PKC may play a central role in this aspect of cell death. For example, PKC has been suggested to play a role in cell cycle control and studies by Fields and co-workers have identified PKC-β1(ll) as a mitotic lamin kinase (Goss et al., 1994). Moreover, phosphorylation of lamin B by PKC was shown to contribute to the disassembly of the nuclear lamina at mitosis. Whether phosphorylation of lamin proteins occurs during apoptosis has not been established, though the ultimate fate of these proteins is proteolytic degradation (Earnshaw, 1995). If a PKC isoenzyme is involved in lamina disassembly during apoptosis, it is unlikely to be PKC-β, as the majority of data support an anti-apoptotic role for this isoenzyme (see earlier). Our own preliminary data have shown an association of PKC-δ with nuclear lamins prior to apoptosis (Lord et al., 1995). Bearing in mind the evidence in favour of a role of this isoenzyme in the induction of apoptosis, these observations now require further consideration. Two tumour suppressor proteins that have been identified as PKC substrates are p53 and bcl-2. p53 is a tumour suppressor protein which is activated in response to DNA-damage, inducing a G1/S stage growth arrest (Donehawer and Bradley, 1993). p53 is phosphorylated at several sites in vivo and by various protein kinases in vitro, including PKC-α and β (Meek, 1994), leading to activation of the DNA-binding function of p53. However, studies in SV3T3 cells suggest that p53 and PKC do not actually interact in vivo to effect DNAbinding of p53 (Milne et al., 1996). Whether PKC may affect other functions of p53 has yet to be determined. Bcl-2-alpha is serine phosphorylated during suppression of apoptosis in growth factor dependent-haemopoietic cells by interleukin-3, or the PKC activator bryostatin-1 (Mays et al., 1994). PKC inhibitors prevent the hyperphosphorylation of bcl-2alpha and the suppression of apoptosis (Mays et al., 1994). Interestingly, these authors also reported that purified PKC was able to phosphorylate bcl-2alpha in vitro in a calciumdependent manner, suggesting that the regulation of bcl-2-alpha was mediated by c-PKCs. These examples illustrate the relevance of studies aiming to identify not only the PKC isoenzymes involved in regulation, but also their physiologically relevant substrates. PKC, APOPTOSIS AND DISEASE As stated at the beginning of this article, PKC has become of major interest in recent years as a target for therapeutic intervention in a range of different diseases. Several of these studies have centred on the role of PKC in the regulation of apoptosis. The removal of a diseased cell by inducing apoptosis has many advantages, not least that apoptotic cells are efficiently removed by phagocytosis, preventing the release of potentially toxic cell contents. Whilst the emphasis in the literature is given to apoptosis and cancer, other disease states clearly include disregulation of apoptosis as a significant pathogenic factor (reviewed in Deacon et al., 1997). These diseases include: disorders of the immune system such as chronic granulomatous disease, in which neutrophil (Coxon et al., 1996) apoptosis is reduced; neurodegenerative disorders, including Alzheimer’s disease, which involves increased apoptosis of neuronal cells (Jenner and Olanow, 1996); vascular disease (Hamet et al., 1996); AIDS (Gougeon, 1996) and Ataxia Telangiectasia (Meyn, 1995) which are both associated with accelerated lymphocyte apoptosis. Alterations in PKC expression or function have been established in several of these diseases, for example, in the case of Alzheimers disease, decreased expression of PKC-β


(Chachin et al., 1996), PKC-ε (Matsushima et al., 1996) and -α and -ζ (Bergamaschi et al., 1995; Govoni et al., 1996; Greenwood et al., 1996) have been reported. However, with the exception of cancer and Ataxia Telangiectasia, the connecting factor between changes in PKC and disease pathogenesis does not appear to involve apoptosis. For example, defective secretion of amyloid-β protein has now been identified as the function directly affected by the alteration in PKC-α in Alzheimers disease (Bergamaschi et al., 1995). However, it is still likely that future studies that consider alterations in specific PKC isoenzymes in relation to disease states and apoptosis may reveal more positive connections. For example, chronic granulomatous disease (CGD) is associated with defective neutrophil apoptosis and activation, with the latter associated with reduced PKC activity (Curnutte et al., 1994). In the case of neoplasia, the role for PKC in disease progression is better defined, though relatively few studies have considered individual PKC isoenzymes. For this reason, and the vast nature of the general literature concerning PKC and cancer, we have chosen to highlight only one cancer in which the role for PKC isoenzymes has been well documented. Colorectal cancer (CRC) is the second most common malignancy occurring in the Western world. Incidence rates for this disease vary 20-fold world-wide, from 25–35 per 100 000 in North America and Western Europe, to 1–3 per 100 000 in India (Potter, 1995). This variation is just one indication that a combination of genetic (Fearon and Vogelstein, 1990) and environmental (Nagengast et al., 1995) factors play an important role in the progression of CRC. Epidemiological data indicate that diets which are high in fat give an increased risk of CRC (Reddy and Wynder, 1977). Bile acids, which are elevated by such diets, are raised in patients with CRC (Reddy and Wynder, 1977). Bile acids have been shown to activate PKC in vitro, with the secondary bile acids showing selectivity in their activation of individual isoenzymes (Pongracz et al., 1995a). PKC-β and ε were activated most effectively by the secondary bile acids deoxycholic acid, ursodeoxycholic acid and lithocholic acid. In contrast, PKC-α was only activated by primary bile acids. Importantly, it is the secondary bile acids which show the greatest increase in patients with CRC (Imray et al., 1992). Many studies have compared total PKC levels in normal and neoplastic colonic mucosa and have found consistently that levels are lower in cancerous tissue (Levy et al., 1993; Doi et al., 1994; Pongracz et al., 1995a). Analyses of PKC isoenzymes at the level of mRNA (Doi et al., 1994) and protein (Pongracz et al., 1995a) have shown a decrease in PKC-β, leading to the suggestion that raised levels of secondary bile acids within the colon lead to persistent activation and ultimate downregulation of specific PKC isoenzymes, notably PKC-β (Pongracz et al., 1995a). As this isoenzyme appears to play a central role in the co-ordinated regulation of proliferation and apoptosis (see previous sections), its reduction in the colonic epithelium may be a key factor leading to the disruption of the normal balance between cell proliferation and loss that has been noted for CRC (Sinicrope et al., 1996). CONCLUSIONS It has not been possible in this review to cover all the literature concerning PKC and its involvement in apoptosis, though certain conclusions can be drawn from the published data at this time. Thus, we propose that PKC isoenzymes are differentially involved in the regulation of apoptosis and whilst there are likely to be variations between cell types, the literature at present suggests that PKC-δ is involved in the execution of the apoptotic


programme. In contrast, PKC-α and -ζ are frequently associated with cell survival and suppression of apoptosis. The role of PKC-βI and -βII remains to be clarified, though both are intimately associated with signalling pathways that modulate cell proliferation and apoptosis. Clearly the role of PKC in apoptosis and disease will only be fully understood when individual family members are considered. Access to specific inhibitors and activators of PKC isoenzymes will also improve our understanding of PKC isoenzyme function and may additionally uncover therapeutically useful compounds (Ishii et al., 1996). In addition, the increased use of specific antisense oligonucleotides and constitutively active PKC isoenzyme mutants, mentioned briefly in this review, has already begun to increase understanding of PKC isoenzyme function. Studies of PKC function such as these, will allow informed decisions to be made regarding the suitability of PKC as a target for drug therapy. ACKNOWLEDGEMENTS EMD, JP and LM are supported by grants from the Leukaemia Research Fund, BBSRC and MAFF, respectively. HC is supported by the United Birmingham Hospitals Endowment Fund. GG holds an MRC PhD scholarship and JML is a Royal Society University Research Fellow. REFERENCES Barr, L.F., Campbell, S.E. and Baylin, S.B. (1997) Anti-protein kinase C β2 inhibits cycling and decreases c-myc-induced apoptosis in small lung carcinoma cells. Cell Growth Differ., 8, 381–392. Barry, S.T. and Critchley, D.R. (1994) The Rho A-dependent assembly of focal adhesions in Swiss 3T3 cells is associated with tyrosine phosphorylation and the recruitment of both PP125 FAK and protein kinase C-delta to focal adhesions. J. Cell Sci., 107, 2033–2045. Baudier, J., Delphin, C., Grunwald, D., Knochbin, S. and Lawrence, J.J. (1992) Character isolation of the tumour suppressor protein p53 as a protein kinase C substrate and a s100bbinding protein. Proc. Natl. Acad. Sci. USA, 89, 11627–11631. Bauer, P.I., Farkas, G., Buday, L., Mikala, G., Meszaros, G., Kun, E. and Farago, A. (1992) Inhibition of DNA binding by the phosphorylation of poly ADP-ribose polymerase protein catalysed by protein kinase C. Biochem. Biophys. Res. Comm., 187, 730–736. Beaver, J.P. and Waring, P. (1994) Lack of correlation between early intracellular calcium ion rises and the onset of apoptosis in thymocytes. Immunol. Cell. Biol., 72, 489–499. Bergamaschi, S., Binetti, G., Govoni, S., Wetsel, W.C., Battaini, F., Trabucchi, M., Bianchetti, A. and Racchi, M. (1995) Defective phorbol ester-stimulated secretion of beta-amyloid precusor protein from Alzheimers-disease fibroblasts. Neuroscience Lett., 201, 1–4. Berra, E., Diaz-Meco, M.T., Lozano, J., Frutos, S., Municio, M.M., Sanchez, P., Sanz, L. and Moscat, J. (1995) Evidence for a role of MEK and MAPK during signal transduction by protein kinase C-zeta. EMBO J., 14, 6157–6163. Borner, C., Filipuzzi, I., Wartmann, M., Eppenberger, U. and Fabbro, D. (1989) Biosynthesis and posttranslational modifications of protein kinase C in human breast cancer cells. J. Biol. Chem., 264, 13902–13909.


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6. APOPTOSIS IN DROSOPHILA JOHN M.ABRAMS†, PO CHEN, WILLIAM NORDSTROM AND JOHNSON VARKEY Department of Cell Biology and Neuroscience, The University of Texas Southwestern Medical Center, 5323 Harry Hinds Boulevard, Dallas, Texas 75235–9039

KEY WORDS: reaper, grim, head involution defective (hid), caspase, IAP, p35.

INTRODUCTION The elimination of cells by apoptotic cell death seems to be a universal feature of development and aging in metazoans. How are otherwise healthy cells condemned to die and what is the mechanism underlying the execution of this selfdestruct program? Approaches to these questions at the molecular level have begun to emerge from model genetic systems and several lines of evidence suggest that the physiology underlying cellular suicide is highly conserved (Jacobson et al., 1997; Steller, 1995). These models could therefore facilitate treatments for human diseases that are caused, or exacerbated by, the misregulation of apoptosis. Mutations isolated in the nematode Caenorhabditis elegans provided the first indisputable evidence that “naturally-occurring” cell death was indeed a genedirected process. Loss of function mutations were found that prevented all cell deaths in this organism (Ellis and Horvitz, 1986) suggesting that a common mechanism of killing is utilized among all cells which die in this animal. The cloning of genes corresponding to several of these mutations has provided molecular entry points into at least three essential components of the apoptotic pathway. One gene which normally acts to suppress programmed cell death (PCD) in nematodes, ced-9, shows structural and functional homology to bcl-2, a proto-oncogene which suppresses apoptosis in mammals (Hengartner et al., 1992; Hengartner and Horvitz, 1994). Another nematode gene, ced-3, is the founding member of a growing family of proteolytic enzymes (Yuan et al., 1993) or Caspases, some of which contribute essential roles during apoptotic cell death throughout the animal kingdom. A third locus, ced 4, links activities of the upstream ced9/bcl2 family to downstream death caspases (Chinnaiyan et al.,

† Corresponding Author: Tel.: 214–648–9226. Fax: 214–648–8694. [email protected] Supported in part by a grant from the NIH: R01 12466

e-mail:

178 J.M.ABRAMS, P.CHEN, W.NORDSTROM AND J.VARKEY

1997; Spector et al., 1997; Wu et al., 1997) and biochemical evidence suggests that this function is conserved as well. PCD is also a common feature in Drosophila development, yet in contrast to the nematode, many cell deaths in the fly are not strictly predetermined by lineage (Abrams, 1996; Abrams et al., 1993; Wolff and Ready, 1991). Development in the fruit fly exhibits remarkable plasticity, including the ability to eliminate cells (by apoptosis) that are either damaged or unable to complete their differentiation program (Abrams, 1996; Abrams et al., 1993). Drosophila is thus uniquely suited for studying how cell interactions or environmental stresses can specify or trigger the cell death fate. This chapter offers a brief overview of studies documenting PCD in Drosophila and reviews more recent findings that focus upon the molecular basis of apoptosis during normal and abnormal development. THE OCCURRENCE OF PCD DURING DEVELOPMENT Cell death in Drosophila has been described in many tissues at virtually all developmental stages. Investigators have generally utilized histological stains (such as toluidine blue) on fixed preparations and/or vital stains on live preparations to selectively identify dying cells. The vital dye acridine orange (AO) can be used to visualize cell death in dissected imaginal discs (Spreij, 1971; Wolff and Ready, 1991; Wolff and Ready, 1993) and, with some adaptation, AO or nile blue can be used to visualize apoptosis in the embryo (Abrams et al., 1993). In fixed preparations, PCD can be visualized by TUNEL labeling (Gavrieli et al., 1992) which exploits the characteristic degradation of chromatin in apoptotic nuclei. The application of these methods to Drosophila tissue suggests that mechanisms responsible for DNA degradation during apoptosis are conserved between vertebrates and invertebrates. Although many of descriptions of PCD in Drosophila occurred prior to common usage of the term “apoptosis”, an examination of published ultrastructure often shows that the mode of death is clearly apoptotic (nuclear material condenses, cells become shrunken and cellular fragments are engulfed). For instance, an examination of PCD in ovarian chambers reported that the nuclei of some nurse cells were severely condensed and the resulting cellular debris was engulfed by neighboring follicle cells (Giorgi and Deri, 1976). Some of the earliest descriptions of cell death in Drosophila were reported by Fristrom (Fristrom, 1968; Fristrom, 1969) studying imaginal development of the eye and wing disc. These reports established that reproducible patterns of inappropriate cell death accounted for structural defects in the adults of a variety of mutant strains and, in essence, were among the first to establish that a mutant genotype could have predictable effects upon the incidence of cell death during development. PCD is a very prominent feature of embryonic development (Campos-Ortega and Hartenstein, 1985) and, at the ultrastructural level, these cell deaths are strikingly similar to apoptotic deaths observed in vertebrate systems (Abrams et al., 1993). Patterns of cell death in the embryo are dynamic and widespread among many different organs and tissues, yet a fairly stereotypical distribution and number are observed for each stage. After embryogenesis, conspicuous numbers of dying cells are not observed until metamorphic changes are initiated. In fact, the widespread histolysis of larval tissue during metamorphosis is often cited as a classic example of PCD (Bodenstein, 1950).

APOPTOSIS IN DROSOPHILA 179

TREATMENTS THAT INDUCE APOPTOSIS Ionizing radiation is the most commonly used agent to induce apoptosis in Drosophila. Irradiation has historically been used to generate mosaic animals during early larval development (Ashburner, 1989) and studies to characterize the biological consequences of these treatments cited clear effects upon the incidence of cell death. In wing discs, treatment with ionizing radiation showed evidence of both apoptosis and necrotic death (Abbot, 1983) whereas only apoptotic deaths were observed in embryos exposed to similar treatments (Abrams et al., 1993; White et al., 1994). Not surprisingly, the stages and cell types that are most vulnerable to radiation correlate with replicative potential (Fryxell and Kumar, 1993; Wurgler and Ulrich, 1976). Moreover, notable differences in the kinetics of apoptosis were associated with exposure doses (Abrams et al., 1993). Induction of cell death by ionizing radiation may reflect a process of “altruistic suicide” or “cell-replacement repair” whereby the elimination of cells that may harbor damaging mutations stimulates the proliferation of healthy cells to replace them (Kondo, 1988). SIGNALS THAT GOVERN CELL DEATH Among several potential endocrine regulators of PCD in Drosophila, the ecdysteroid hormones are the most well understood. Regulation of cellular physiology and gene expression cascades by steroids are extremely well characterized at the molecular and genetic level (Ashburner, 1989; Baehrecke, 1996). Much of our knowledge regarding the regulation of PCD by ecdysteroids derives from analogy to studies of the moth Manduca sexta and the giant silkworm Antheraea polyphemus (Truman, 1984; Truman, 1992) which are particularly amenable to detailed physiological studies because of their large size. In these insects, metamorphic cell death is largely governed by falling titers of ecdysteroids. Withdrawal of this hormone can either directly trigger cell death (Schwartz and Truman, 1983) or, alternatively, lower ecdysone levels can “prime” cells for this fate (Schwartz and Truman, 1982). Because these deaths can be suppressed by cyclohexamide (Farbach and Truman, 1988) or actinomycin D (Truman, 1992) they probably require macromolecular synthesis. Other endocrine factors, including juvenile hormone (Schwartz, 1992) and eclosion hormone (Schwartz and Truman, 1982) have been implicated as cues that elicit PCD in larger insects which might serve similar functions in Drosophila. Mutations in genes that encode receptors for ecdysone are providing exciting avenues for exploring hormonal mechanisms that regulate PCD. Probes that discriminate isoforms of the ecdysone receptor (Koelle et al., 1991) have already provided some tantalizing clues. This receptor encodes three protein isoforms (Talbot et al., 1993) that share DNA and hormone binding domains yet differ in their N-terminal regions. Expression patterns of each isoform show distinct tissue distributions that correlate with differential responses to ecdysone during metamorphosis (Talbot et al., 1993). High expression levels of one isoform, EcR-A, specifically anticipates PCD in a heterogeneous group of cells in the central nervous system (Robinow et al., 1993). Apparently, the predetermined fate of these doomed cells is reflected by preferential expression of this receptor isoform. The functional importance of this observation was substantiated by showing that treatment with ecdysone could block the death of these cells if provided 3 hrs. prior to degeneration. Withdrawal of edysone may


trigger the onset of death in neurons expressing high levels of EcR-A by direct induction of death-related genes such as reaper (see below). Isoform specific differences of these receptors should therefore provide important clues to explain how cells chose to survive or die in response to a common hormonal signals. MUTATIONS THAT CAUSE INAPPROPRIATE APOPTOSIS Lesions at a variety of loci are associated with characteristic patterns of ectopic cell death in many regions of the developing fly. Estimates from one screen for cell death defective mutations suggest that nearly 20% of the genes in the fly genome can cause excessive PCD when mutated (White et al., 1994). A survey of cloned genes associated with ectopic cell death phenotype suggests that most of these function in the specification of cell fate during development (reviewed in (Abrams, 1996)). This correlation suggests that the inability to differentiate properly will often elicit apoptotic responses (Abrams et al., 1993). The fact that so many different mutations can influence the pattern of cell death argues that apoptosis can be the outcome of a default program triggered by conflicting developmental signals. This response may contribute to plasticity during cell fate determination and it is thus difficult to establish whether ectopic death phenotypes reflect secondary consequences of aberrant development or specific apoptosis functions. Mutations displaying the opposite phenotype (see below) are therefore more likely to reveal specific components of the apoptosis machinery. MUTATIONS THAT REDUCE THE INCIDENCE OF APOPTOSIS Some mutations in Drosophila cause a reduction of the incidence of PCD in specific tissues. These loci could affect components necessary to specify cell death, and may be similar to a class of genes in C. elegans that either determine or trigger PCD in the nematode (Ellis and Horvitz, 1986). Mutations at apterous were perhaps the first phenotypes associated with failures in PCD (Butterworth, 1972; Butterworth and King, 1965). In wild type adults, cells of the larval fat body disappear entirely within the first 4 days after emergence. Animals bearing lesions at apterous initially show delayed rates of adipose cell death and, eventually, these animals show a complete block to PCD in these cells. More than 20% of the larval cells that would otherwise have died were found to persist in apterous adults. Several experiments, including cell transplantations, suggested that the PCD phenotype was nonautonomous and possibly associated with endocrine, neuronal or cytolytic factors. Ironically, mutations at this same locus may also cause excessive PCD in other cell types (Sedlak and Manzo, 1984). Because apterous encodes a transcriptional regulator (Cohen et al., 1992) it is possible that these phenotypes arise via the misregulation of secondary agents that trigger PCD. At least three mutations affecting eye development are associated with reduced cell death phenotypes. One allele of Notch reduces the number of cell deaths in the developing retina (Cagan and Ready, 1989). This gene is required for a variety of local cell interactions that specify differentiation fates. PCD phenotypes associated with mutations at this locus could therefore result from misrouting of cell fate decisions rather than a direct failure to die. Two other mutations that disrupt retinal development, roughest and echinus, also appear to reduce


the incidence of PCD. In the case of the former, mosaic analyses suggested that gene action is cell autonomous and may reflect failures in one of many signalling pathways that provoke apoptosis (Wolff and Ready, 1991). Sequence analysis shows that roughest is a large transmembrane protein containing several immunoglobulin-like motifs and that the allele which affects PCD disrupts the intracellular domain (Ramos et al., 1993). There are other genes that affect the rate of cell death. For example, after an adult fly emerges from the puparium, a group of muscles in the head degenerate within 12 hours. Two mcd (muscle cell death) mutations delay this process showing blocks to fragmentation and/or absorption of the muscle cell corpses. PCD in the head muscles of mcd mutants is clearly distinct from wild type, suggesting that these genes might function as part of the machinery which dismantles dying cells (Kimura and Tanimura, 1992). THE REAPER REGION, A GENOMIC INTERVAL REQUIRED FOR APOPTOSIS The reaper (rpr) region is required for all cell deaths which occur during embryonic development in Drosophila (Abrams et al., 1993; White et al., 1994). This locus was identified from a screen which sampled ~50% of the fly genome for cell death defective mutations (White et al., 1994). No apoptotic cell death is observed in embryos deleted in the rpr interval. Although they develop fairly normal segmentation and cuticular structures these individuals ultimately show failures in head involution and nerve cord condensation and they do not hatch as larvae. Some of these defects probably reflect the anatomic consequences of failures in cell death commencing at earlier developmental stages. Several lines of evidence argue that functions mapping to the rpr region are globally required PCD during Drosophila development. First, direct inspection using multiple histological methods (including electron microscopy and TUNEL labeling) shows a conspicuous absence of apoptotic cells. Second, the macrophages in rpr mutants are unusually small and devoid of internalized cell corpses with which they normally are engorged. Third, extra cells (that would otherwise die) persist in these mutants and they adopt differentiated fates typical of their surrounding tissue (Grether et al., 1995; White et al., 1994; Zhou et al., in press). Together, these observations demonstrate a requirement for this genomic region during normal PCD. Is the reaper interval also involved in cell deaths induced by exogenous agents or developmental defects? The answer to both is affirmative. When embryos deleted for the rpr region are exposed to X-irradiation, the numbers of induced cell deaths are far less than those observed for wild type siblings exposed to the same treatment (White et al., 1994). Therefore, components in the rpr interval may not be part of the “apoptosis machinery” itself, but may instead trigger a set of commonly used effector molecules. This radiation resistant phenotype is reminiscent of cells that lack p53 (Clarke et al., 1993; Lowe et al., 1993) and demonstrates that: 1) loss of genes in the rpr region confer protection against the induction of ectopic death by at least one extrinsic agent and 2) some apoptotic deaths can occur in the absence of these functions. Apparently, ionizing radiation can induce apoptosis through at least two genetic pathways, one of which depends on functions that are utilized during normal embryonic development.


Defective development is often associated with ectopic or extra cell deaths (see above and (Abrams, 1996)) and deletions of the rpr interval will also prevent these (White et al., 1994). For example, mutations at crumbs cause failures in the development of various tissues followed by massive apoptosis among epithelial cells that normally survive. When this mutation was introduced into a genetic background deleted for the reaper interval, the developmental failures still occurred but the associated ectopic deaths were prevented. Collectively, these observations argue that functions in the reaper interval, are not only necessary for PCD during normal development but are also required for abnormal deaths induced either by damaging agents or congenital defects. The cell death phenotype was originally defined by a deletion interval that maps to genomic region 75C1, 2 on the third chromosome (see Figure 6.1). The H99 deficiency is the smallest deletion associated with a fully penetrant cell death defective phenotype and spans ~ 300kb of DNA. Other relevant deletions (X14 and X25) have breakpoints that map within the H99 interval and strains bearing these deletions exhibit cell death defects that are less severe than H99 homozygotes. Interestingly, the penetrance of each is markedly enhanced when placed in trans to the H99 chromosome. The complex genetic properties associated with these deletions led to suspicions that multiple cell death functions might reside in the obligate region, a prediction that has since been confirmed at the molecular level. To identify the corresponding functions at the molecular level, a genomic walk through the obligate interval was used to establish a contig upon which deletion breakpoints and transcripts were mapped. Reaper was the first cell death gene identified in this interval. This gene is selectively expressed in cells that will later die and partially restores PCD to H99 embryos in transformation experiments (White et al., 1994). Moreover, expression of rpr is sufficient to trigger apoptosis in both transgenic animals and cultured cells (Chen et al., 1996a; Nordstrom et al., 1996; Pronk et al., 1996; White et al., 1996). RPR is a 65 amino acid cytoplasmic protein bearing no obvious similarities to known proteins. Alignments between RPR and the death domain of tumor necrosis factor receptor 1 family members have led to suggestions that these proteins might share an ancestral heritage (Cleveland and Ihle, 1995; Golstein et al., 1995a; Golstein et al., 1995b). However, empirical tests have so far failed to obtain evidence supporting the notion that RPR and the TNFR1 death domain share common functions (Chen et al., 1996a; Vucic et al., 1997). Despite extensive efforts, no mutant fly strains bearing single gene lesions in rpr have, as yet, been isolated. The identification of mutations in this gene could be hampered by its rather small size and the occurrence of additional cell death genes in the region that probably serve redundant functions (see below). Robinow and his colleagues have gathered some of the most convincing data in support of the idea that the expression of rpr RNA anticipates the onset of pro grammed cell death. These workers focus on a set of doomed neurons (n4 neurons) which are marked during pupation by high levels of the EcR-A ecdysone receptor isoform (see above) and die 4–8 hr. after eclosion (Robinow et al., 1993). At eclosion, rpr RNA is undetectable in the n4 neurons yet high levels of rpr are observed in these same cells one hour later (Robinow et al., 1993). Moreover, treatments that hormonally block these cell deaths also prevents the appearance of rpr expression. In these cells, the onset of rpr expression correlates with (and might define) a commitment to the death program.


Figure 6.1 Physical map of the Drosophila cell death interval at 75C1,2. Deletion chromosomes are depicted above the genomic interval where each notch represents a physical distance of 100 kilobases. Relevant transcripts (not drawn to scale) are schematically represented below. Left-ward direction is toward the telomere. Dotted lines represent DNA that is missing in each deletion strain and thick bars represent portions that are intact. Strains with dark, solid bars display a fully penetrant cell death defective phenotype. Strain X37, represented by the open bar, shows no cell death phenotype and thus delimits a distal boundary for cell death functions. Strains that display partially penetrant cell death phenotypes, X14 and X25, are denoted with shaded bars. The intensity of shading indicates the severity of the phenotype (darker shading corresponds to increased penetrance; see text). The obligate interval defining the cell death function(s) thus resides inside the breakpoints of the H99 deletion. The transcript encoded by hid represents the only singlegene lethal function identified in this interval.

Studies that monitor expression of rpr in contexts where ectopic cell death is provoked by exogenous damaging agents have also born out predictions of earlier genetic evidence. For instance, expression of rpr (shown in Figure 6.2) is acutely responsive to ionizing radiation such that a robust elevation of this RNA anticipates the onset of X-ray induced apoptosis (Nordstrom et al., 1996). Similar cell specific induction is observed in models of defective development where inappropriate expression of rpr anticipates inappropriate cell deaths (Nordstrom et al. 1996; Singer et al., 1996). The fact that rpr is sufficient to trigger apoptosis, together with observations showing that expression anticipates both programmed and induced cell death, imply that mechanisms regulating the occurrence of this transcript can profoundly influence the decision of a given cell to survive or die. In large measure, this regulation appears to be achieved at the transcriptional level. The behavior of rpr in models of inappropriate cell death is recapitulated by a lacZ reporter transgene (Nordstrom et al., 1996) and, at the gross level this


Figure 6.2 (A). Transactivation of reaper RNA anticipates radiation-induced cell killing. Left column extensive induction of apoptosis 1.5 hrs. after exposure to 4000 rads of γ-rays (R) compared to control (C) is detected by acridine orange staining. Note that longer exposure times for control embryo image (C) results in increased detection of yolk autoflorescence (denoted by arrow). Prominent induction at the rpr promoter is detected by staining for β-galactosidase in embryos bearing a rpr-promoter-lacZ reporter transgene (Rpr11LacZ) 1.5 h after exposure to radiation (R) when compared to untreated samples (C). (B). rpr RNA levels are induced by ionizing radiation. Hybridization of a radio-labeled rprspecific probe to RNA from irradiated embryos (lane R) and similarly staged control embryos (lane C) is shown in the left hand panel. Ethidium bromide staining of the gel (right panel) demonstrates that equivalent quantities of target RNA were assayed. (taken from (Nordstrom et al., 1996))

reporter also mirrors some (but not all) elements of the endogenous expression pattern (see Figure 6.2). These observations raise the possibility that the rpr promoter might define a molecular integration site for a number of distinct death signals. Alternatively, the integration of cues governing cell death might occur upstream of the rpr locus. Discriminating between these possibilities and determining the nature of the circuits that govern transactivation of the rpr locus will be exciting avenues of future exploration. GRIM As the physical map of the reaper region was compiled, evidence suggesting that the H99 interval might harbor more than a single cell death gene began to emerge. The most definitive observation in this regard was that internal deletion strains (DfX14 and DFX25) displayed markedly different phenotypes when placed in trans to H99 (Figure 6.1). The more severe phenotype associated with DfX25 could be explained by presuming that at least one additional cell death function maps between the proximal breakpoints of DfX14 and DfX25. Direct tests for such a function led to the identification of grim (Chen et al., 1996b) a cell death gene which exhibits many features in common with rpr.


Several lines of evidence established that grim encodes an activator of PCD during development (Chen et al., 1996b). First, germline transformation of genomic DNA spanning the grim locus resulted in restoration of cell death to H99 mutant embryos. Although only partial rescue of the cell death defect by the grim cosmid was observed, the extent of rescue was dose dependent and similar to levels of rescue observed for corresponding doses of genomic rpr. Second, induction of grim either in cultured cells or in ectopic tissues triggered extensive apoptosis as determined by methods that are selective for this form of cell death (AO and TUNEL). Third, these deaths were prevented by p35, a viral inhibitor of apoptosis which targets essential components of the death machinery (see below). Fourth, the distribution of grim transcript during embryogenesis is coincident with patterns of embryonic cell death. Moreover, grim RNA was occasionally detected inside phagocytes. This feature is an unusual staining characteristic that has also been observed with probes for rpr (White et al., 1994; Zhou et al., 1995) and hid (Grether et al., 1995) and probably reflects hybridization to RNAs that persist within engulfed cell corpses. Finally, a burst of grim expression precedes the death of n4 neurons and, in this respect, also appears to mirror the behavior of rpr (Robinow et al., 1993). These observations suggest that, like rpr, RNA expressed from the grim gene is a molecular prophet of death. The deduced open reading frame (ORF) for grim is a protein of 138 amino acids with no predicted trans-membrane domains (Rost and Sander, 1994) and no extensive homologies to any sequence in the current database. However, the amino terminal end of GRIM shares very notable similarity to RPR (Chen et al., 1996b). Ten of the first 14 residues of GRIM are identical to the corresponding position in RPR while three of the remainder are conserved substitutions. Interestingly, although a RPR mutant deleted for these residues is a less stable protein, it is still a fairly proficient activator of death in cell culture assays. This shared N terminal motif is therefore not essential for killing functions (at least with respect RPR) (Chen et al., 1996a) and its functional significance remains somewhat obscure. Studies on epitope-tagged versions of this protein suggest that GRIM is predominantly localized to the cytoplasmic compartment (Varkey and Abrams, unpublished observations). HEAD INVOLUTION DEFECTIVE At least two mutagenic assaults on the H99 cell death interval suggest that head involution defective (hid) is the only lethal complementation group accessible by chemically induced point mutations in this region (Abbott and Lengyel, 1991; Grether et al., 1995; White et al., 1994). The hid gene was originally described as a locus required for proper head and genitalia formation (Abbott and Lengyel, 1991). Embryos carrying single gene hid mutations fail to complete the final morphogenetic movements of the anterior head segments and the overwhelming majority of these embryos do not hatch as larvae. The rare hid individuals that do survive to adulthood (~ 5% of expected frequency) exhibit defects of the genitals and wing blades. hid mutations do not exhibit gross cell death defects that phenocopy H99 but they do manifest partial failures in apoptosis (Grether et al., 1995). The incidence of PCD (particularly in the developing head region) is noticeably decreased in these mutants and supernumerary cells are observed in larval optic nerve. Singlegene null alleles of hid cause phenotypes that are relatively mild when compared to the severity of defects associated with


the H99 deletion interval. This difference is emphasized by the fact that null alleles of hid are only semi-lethal whereas H99 is an absolutely lethal deletion (all homozygous embryos die). These observations, and the complex complementation pattern of hid point mutations, establish that the hid gene function alone does not fully account for the severity apoptosis defects associated with 75C1, 2 deletions. Nevertheless, it is also clear that expression of hid is sufficient to trigger extensive apoptosis in both transgenic animals (Grether et al., 1995) and cell culture (Varkey and Abrams, unpublished observations). Also, like the previous models described for rpr and grim, these deaths are also prevented by co-expression of p35. The predicted hid gene product is a novel 410-amino-acid protein rich in serine and proline residues. Although hid mRNA occurs in many regions that are coincident with patterns of PCD, it does not behave like a “prophet of death” because expression of this gene occurs in cells that do not ultimately die and it is absent from at least one organ (the nerve cord) at a time when substantial PCD occurs there. GENETICS OF THE H99 CELL DEATH INTERVAL At least three cell death genes are uncovered by the H99 deletion. These share a common, short motif at the N terminus and are all transcribed in the same orientation (towards the telomere). Are there additional cell death genes in the H99 interval? Genetic approaches to this problem are hampered by the absence of single-gene mutations in rpr or grim. The fact that only alleles of hid were recov ered from screens for lethal mutations in the H99 interval (Abbott and Lengyel, 1991; Grether et al., 1995; White et al., 1994) could suggest that null mutations in rpr or grim might be even more mild than hid and could, therefore, be inaccessible by screens for lethal or visible phenotypes. According to this scenario, hid, rpr and grim might encode partially redundant functions and the more severe (or noticeable) cell death phenotypes could require the disruption of at least two of the three known elements in the region. Conversely, if these three loci accounted for all apoptosis functions in the reaper region, simultaneous restoration of both rpr and grim functions might be expected to convert H99 towards a phenocopy of hid null mutations. Tests of this prediction, however, were negative. H99 embryos homozygous for grim and rpr cosmid-transgenes do not phenocopy hid with respect to levels of PCD (these individuals are indistinguishable from H99 embryos bearing four doses of either grim or rpr). However, since expression from cosmidtransgenes might be compromised, solid conclusions regarding the number of distinct cell death functions uncovered by H99 can not be drawn from these results. In Drosophila, and in other species, tight linkage can occur among groups of genes that share closely related functions (Krumlauf, 1994; Lawrence, 1992). Typically, the individual members within conserved gene clusters share a common orientation of transcription and a considerable degree of sequence similarity. The H99 cell death interval therefore, shares some, but not all, of the classic features of a complex: grim, hid and rpr share commonality of orientation and function yet except for a short motif at the N terminus, these proteins share little or no sequence similarity. Since individual members within a gene cluster are typically thought to have arisen by evolutionary duplication and divergence (Kenyon, 1994), it is possible that organization of the H99 cell death cluster may have evolved by different mechanisms.


How do these genes function to elicit apoptosis? Investigating this question is certain to be a high priority for the future yet already several observations permit us to make some predictions in this regard. First, conditional expression of each is sufficient to elicit cell death in H99 homozygotes: rpr-mediated apoptosis does not require grim or hid, grimmediated apoptosis does not require rpr or grim and hidmediated apoptosis does not require grim or rpr. Thus rpr, grim and hid can each function independently to trigger apoptosis. Moreover, experiments that test for cross-regulation among these genes have obtained no evidence for feedback circuits at the level of gene expression (Chen et al., 1996b; Grether et al., 1995). However, at least one example occurs where rpr and hid cooperate to trigger apoptosis when neither alone was sufficient (Zhou et al., in press). Therefore, although each can function independently to trigger apoptosis, there will certainly turn out to be combinatorial requirements and selective constraints that specify patterns of PCD in different contexts. A second important consideration is that death pathways triggered by either RPR, GRIM or HID are all similarly influenced by Drosophila homologs of the IAP genes, DIAP1 and DIAP2 (see below). Third, these activators of apoptosis proceed through a p35 inhibitable step, implicating a downstream requirement for Caspase function (see below). These gene products might thus be viewed as parallel switches that might ultimately activate a common circuit of apoptotic downstream effectors (see Figure 7.3). Alternatively, two or more parallel pathways, each of which is separately blocked by p35, could define the effector circuits downstream of these genes. In either case, grim, rpr and hid might represent alternate switches that are triggered by distinct, but overlapping, sets of apoptotic signals. These activators of death are, for the most, indistinguishable with respect to their activities in transgenic animals and cell culture assays, Yet, in at least one context, the functions of RPR and GRIM are not identical-ectopic grim induction triggers apoptosis at a time in early embryonic development when no such effects were observed with rpr (Chen et al., 1996b). This distinction could reflect differences in cell death signalling that are uncovered during early development. For instance, a negative regulator of rpr that does not block grim might be present at this stage or, alternatively, an effector required for rpr but not for grim might be absent at this stage. THE DROSOPHILA IAP GENES The first members of the IAP (Inhibitors of Apoptosis) gene family were identified as baculoviral proteins that function to suppress the death of infected host cells (Clem and Miller, 1994). These viral IAPs were probably acquired from host genomes as an evolutionary strategy that maximizes productive replication. IAPs typically encode a RING finger motif at their carboxy terminus and one or more BIRs (Baculovirus IAP repeats) at the amino terminus (Duckett et al., 1996; Hawkins et al., 1996; Liston et al., 1996; Uren et al., 1996). The former may constitute a negative regulatory domain (Hay et al., 1995), whereas the latter motif is essential for the anti-apoptotic activity associated with these proteins (Clem and Miller, 1994; Duckett et al., 1996; Hawkins et al., 1996; Hay et al, 1995; Uren et al., 1996). The mechanism by which these proteins prevent apoptosis is not yet known. Members of the IAP gene family occur in mice and humans and at least two have been identified in Drosophila.


Figure 7.3 Alternative models of gene action for functions in the reaper region. Signals governing transcriptional activation (open arrows) are thought to regulate rpr, grim and hid expression (proven thus far only for rpr). These gene products might converge upon a singular hypothetical circuit that can be blocked by IAPs and p35 (left side) or might operate in parallel circuits, each of which can be separately blocked by IAPs and p35 (right side). Death induced by these activators ultimately triggers activation of Caspases and cleavage of DNA. Known functions are solid arrows, hypothetical ones are dotted. Some observations suggest that grim may engage the death circuitry further downstream than rpr.

In a screen for modifiers of rpr-mediated apoptosis in the eye, Hay and his colleagues identified mutations at DIAP1 (Hay et al., 1995). Mutations at this locus are dominant enhancers of the death phenotype and were determined to be allelic to a previously identified gene referred to as thread (th). Sequence analyses prompted by the discovery of DIAP1 led to the identification of a related gene, referred to as DIAP2. Unlike DIAPl, deletions uncovering DIAP2 do not enhance rpr-mediated retinal cell death. Overexpression of DIAP1 or DIAP2 not only suppressed normal PCD in the eye, but also prevented excessive apoptosis due to overexpression of rpr, hid (Hay et al., 1995) or grim (Chen and Abrams, unpublished observations). What is the precise function of the IAP proteins during normal development? In situ hybridization experiments show that expression of DIAP1 is widespread throughout the embryo and the developing eye disc. However, embryos homozygous for loss-of-function mutations in either DIAP1 or DIAP2 showed no obvious cell death phenotypes, possibly owing to extensive maternal contributions of this product and/or functional redundancies. Results from clonal analyses in eye discs or ovaries suggest that DIAP1 could be required for cell survival, but the results could alternatively suggest a requirement for proliferation.


Therefore, although the Drosophila IAP genes products exhibit potent activities in sensitized transgenic backgrounds, their function in normal development has yet to be determined. PREVENTION OF APOPTOSIS BY P35 p35 is encoded by the Autographa californica nuclear polyhedrosis virus (AcMNPV) which normally infects lepidopteran insect cells. In a series of elegant studies (Clem and Miller, 1994; Clem et al., 1991). L. Miller and her colleagues established that this protein functions to suppress apoptosis that would otherwise occur among infected host cells. p35 bears no obvious similarity to known proteins yet it is capable of suppressing cell death in a wide range of heterologous contexts. In part, p35 prevents apoptotic death because it is an irreversible inhibitor of (and pseudosubstrate for) members of the ICE/CED-3 (Caspase) family of apoptotic proteases (Bertin et al., 1996; Bump et al., 1995; Xue et al., 1996). Activation of Caspases may be a universal feature of apoptotic cell death and numerous studies argue that Caspases are essential effectors of the apoptotic pathway (reviewed in (Alnemri, 1997; Chinnaiyan and Dixit, 1996; Kumar, 1995; Nicholson, 1996)). The importance of these proteins as requisite components in apoptotic circuits is further bolstered by the fact that viral products (e.g. p35 and crmA) target these enzymes for inactivation (reviewed in (Teodoro and Branton, 1997)). In both transgenic flies and cell culture models, p35 is a potent death suppressor in models of rpr, grim or hid—induced apoptosis (Chen et al., 1996b; Grether et al., 1995; Nordstrom et al., 1996; White et al., 1996). More striking, perhaps is the observation that expression of p35 in wild type embryos can phenocopy deletions of the entire rpr region (Hay et al., 1994). These observations suggest that p35 intervenes downstream of functions in the reaper region and also imply that death triggered by rpr, grim and hid operates through Drosophila members of the Caspase gene family. DROSOPHILA CASPASES Cell killing induced by RPR, GRIM or HID triggers induction of proteolytic activity characteristic of members of the Caspase gene family. This activity is strikingly similar to mammalian counterparts because extracts from Drosophila cultures exhibit signature cleavage activity against the mammalian death substrate, polyADP-ribose-polymerase (PARP) (Chen and Abrams, unpublished observations). At least three members of the Caspase family have been identified thus far in Drosophila. Two of these, DCP-1 (Song et al., 1997) and drICE (Fraser and Evan, in press), are most closely related to CPP-32 and Mch2, whereas the third, DREDD (Chen et al., in preparation), is most closely related to CED-3 and ICH-1/NEDD. A precise determination of the role of these enzymes during PCD will require detailed genetic and biochemical analyses. Precedents from genetic studies on DCP-1 (Song et al., 1997), however, suggest that sophisticated analyses for characterizing multiply-mutated individuals will be required because PCD failures are not observed in embryos homozygous for DCP-1 loss-of-function mutations (these mutations cause larval lethality). Functional redundancy and/or maternal contributions of this gene product might account for the fact that zygotic DCP-1 is


dispensable for PCD in the embryo. Larva homozygous for dcp-1 show no direct failures in PCD defects. These individuals lack all imaginal discs and gonads and exhibit evidence of unusual self-reactive immune responses. ENGULFMENT OF APOPTOTIC CELLS Engulfment of dead cell corpses is the final stage of apoptotic death throughout the animal kingdom. The “professional phagocytes” in Drosophila are referred to as macrophages or hemocytes. Although these cells are most frequently observed to engulf late stage corpses, they display a remarkable capacity to recognize and discriminate dying cells, even at very early stages of apoptosis (Abrams et al., 1993). Macrophages appear to be a homogeneous group of cells which derive from the mesoderm of the head prior to, and independently of, the first signs of PCD (Tepass et al., 1994). Because abundant cell death can be observed in mutations that lack macrophages, these cells are apparently not essential for cell killing in the embryo (Tepass et al., 1994). There are no Drosophila mutations yet described that show specific failures in the engulfment process but at least two genes have been suggested as candidate functions in this process. The class C scavenger receptor, dSR-CI, is specifically expressed in Drosophila macrophages and might contribute to the recognition and processing of dying cells (Abrams et al., 1992; Pearson et al., 1995). Scavenger receptors endocytose a wide variety of polyanionic macromolecules (Krieger and Herz, 1994) and have been implicated in the recognition of apoptotic cells (Abrams et al., 1992; Krieger and Herz, 1994; Savill et al., 1993). A potential mechanism to account for the recognition of apoptotic cells involves phophatidylserine, a ligand for macrophage scavenger receptors (Nishikawa et al., 1990) which is restricted to the inner monolayer in healthy cells. This asymmetric distribution is lost during apoptotic death, and may be recognized on the surface by one or more receptors (Fadok et al., 1992), including the macrophage scavenger receptors. The recovery of dSR-CI mutant strains will permit a rigorous test of its function in the engulfment process. Another gene that has been proposed to play a role in the recognition of apoptotic by macrophages is peroxidasin, a unique heme peroxidase that appears to be deposited in the extracellular matrix (Nelson et al., 1994). This protein, originally referred to as “protein X” (Abrams et al., 1993; Fessler and Fessler, 1989), is synthesized by macrophages and may have both intracellular and extracellular oxidation activity (Nelson et al., 1994). These properties have led to the intriguing proposal that peroxidasin is anchored to basement membranes and could thus function to “mark” damaged or dying cells that might secrete H2O2 or other peroxidase substrates (Nelson et al., 1994). Cells “marked” in this way could be recognized for disposal by scavenger receptors. This proposal is attractive because it relies upon multiple activities for the detection of apoptotic cells which could impart both high selectivity and fidelity to the recognition process. FINAL REMARKS During the past several years, apoptosis research has witnessed spectacular growth. A general lesson to be drawn from our knowledge thus far is that the parallels between apoptosis in Drosophila and mammals are striking. In both systems, the death of cells is


regulated by hormones, occurs in reproducible patterns in development and is associated with a characteristic cytomorphology. Similarly, in both systems, ectopic apoptosis can be observed in association with defective development and induced by exposure to damaging agents. These parallels persist at the molecular level as well. In both systems, cell death is associated with Caspase activity, DNA degradation and modulation by IAPs. Although our knowledge of apoptosis in Drosophila is still rudimentary, the prospects for rapid progress in this area are outstanding. Persuasive evidence argues for a high degree of evolutionary conservation in components required for PCD, and it is therefore likely that understanding apoptosis in Drosophila will have profound and direct relevance to the understanding and treatment of various human pathologies. Genes homologous to rpr, grim and hid, for instance, could provide excellent targets for the discovery of therapeutic drugs that are intended to block or induce apoptosis in humans. REFERENCES Abbot, L.A. (1983) Ultrastructure of cell death in Gamma-or X-irradiated imaginal wing discs of Dro-sophila Radiat. Res., 96, 611–627. Abbott, M.K. and Lengyel, J.A. (1991) Embryonic head involution and rotation of male terminalia require the Drosophila locus head involution defective. Genetics, 129, 783–9. Abrams, J.M. (1996) Molecular and Genetic Control of Apoptosis in Drosophila. In “Apoptosis in Normal Development and Cancer” (M., Sluyser, Ed.), pp.171–188. Taylor & Francis, London. Abrams, J.M., Lux, A, Steller, H. and Krieger, M. (1992) Macrophages in Drosophila embryos os and L2 cells exhibit scavenger receptor-mediated endocytosis. Proc. Nat’l. Acad. Sci. USA, 89, 10375–10379. Abrams, J.M., White, K., Fessler, L. and Steller, H. (1993) Programmed cell death during Drosophila embryogenesis. Development, 117, 29–44. Alnemri, E.S. (1997) Mammalian cell death proteases—a family of highly conserved aspartate specific cysteine proteases.Journal of Cellular Biochemistry, 64, 33–42. Ashburner, M. (1989) “Drosophila: A Laboratory Handbook.” Cold Spring Harbor Laboratory Press, Cold Spring Harbor. Baehrecke, E.H. (1996) Ecdysone signalling cascade and regulation of Drosophila metamorphosis. [Review] [69 refs]. Archives of Insect Biochemistry & Physiology, 33, 231–44. Bertin, J., Mendrysa, S.M., Lacount, D.J., Gaur, S., Krebs, J.F., Armstrong, R.C., Tomaselli, K.J. and Friesen, P.D. (1996) Apoptotic suppression by baculovirus p35 involves cleavage by and inhibition of a virus-induced ced-3/ice-like protease.Journal of Virology, 70, 6251–6259. Bodenstein, D. (1950) The Postembryonic Development of Drosophila. In “Biology of Drosophila” (Demerec, Ed.), pp. 275–364. John Wiley & Sons, Inc, New York. Bump, N.J., Hackett, M., Hugunin, M., Seshagiri, S., Brady, K., Chen, P., Ferenz, C., Franklin, S., Ghayur, T., Li, P., Licari, P., Mankovich, J., Shi, L.F., Greenberg, A.H., Miller, L.K. and Wong, W.W. (1995) Inhibition of ICE family proteases by baculovirus antiapoptotic protein p35. Science, 269, 1885–1888. Butterworth, F.M. (1972) Adipose tissue of Drosophila melanogaster. V., Genetic and experimental studies of an extrinsic influence on the rate of cell death in the larval fat body. Dev. Biol., 28, 311–325.


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7. BACULOVIRAL LESSONS IN APOPTOSIS CHRISTINE J.HAWKINS*†, ELIZABETH J.COULSON AND DAVID L.VAUX *Caltech Division of Biology 156–29, 1201 East California Blud., Pasadena, CA 91125, USA

Apoptosis is an evolutionarily conserved process of fundamental importance that is used in multicellular organisms for development, homeostasis and defence. Cells often respond to infection by intracellular pathogens by undergoing apoptosis, thus limiting replication of the pathogen and spread of the infection. Viruses have evolved that carry anti-apoptotic genes to disable this response. In baculoviruses, two families of such cell survival genes have been found that encode the p35 proteins and the inhibitor of apoptosis (IAP) proteins. p35 inhibits apoptosis by binding to activated caspases, the key apoptosis effector proteases. How lAPs work has not been determined, but their existence in viral genomes as well as those from insects and mammals indicates their function is highly conserved. One member of this family has been implicated in an inherited neurodegenerative disease, and it is possible that other similar diseases may result from lesions in other members of this family. By studying these viral anti-apoptotic genes and their cellular relatives, much can be learnt not only about the control and implementation of the apoptotic process, but hopefully also about the clinical manifestations of abnormal regulation of apoptosis.

INTRODUCTION Functions Accomplished by Apoptosis Apoptosis is a physiological process used to eliminate unwanted cells. It is used during development, to sculpt the body. In adult organisms, it fulfils a homeostatic role, to regulate numbers of cells in organs and tissues by balancing mitotic cell production. Apoptosis is also † Corresponding Author: Tel.: 16263956451. Fax: 1626449 0756. e-mail: [email protected]

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used as a defence mechanism. Cell suicide can provide protection both against dangerously altered cells of the organism itself, as well as against intracellular pathogens such as viruses. The ability of a virally infected cell to detect the infection and respond by killing itself limits the ability of the virus to replicate and hence protects the organism as a whole from a more widespread infection. As a counter strategy some viruses carry genes which interfere with the ability of infected cells to activate apoptotic pathways, thus allowing the virus to replicate and spread to other cells. This is evident from experiments in which such genes are mutated, resulting in reduced infectivity of the virus due to enhanced death of infected cells. Such a process of altruistic apoptosis is most readily understood evolutionarily in the context of multicellular organisms, although there are reports of similar phenomena occurring in single cellular organisms where genetically related cells nearby may be saved by an infected cell altruistically killing itself (Shub, 1994, Yarmolinsky, 1995,Cornillon et al., 1994). VIRAL ANTI-APOPTOTIC GENES Apoptosis is used as an anti-viral defence, and as one might expect, some viruses have evolved strategies which enable them to evade this host response. Various viruses have adopted different ways of achieving this. Some, such as Epstein Barr Virus, African Swine Fever Virus, and Kaposi sarcoma associated Human Herpes Virus 6 carry genes which resemble bcl-2, both in sequence and function (Henderson et al., 1993; Neilan et al., 1993; Cheng et al., 1997; Szrid et al., 1997). A second molecular tactic used by viruses to foil their host’s apoptotic response to infection is to carry genes encoding proteins which interfere with components of the cell death machinery, such as the ICE/ced-3 like proteases. These key effector proteases of apoptosis have been termed caspases because they are cysteine proteases which cleave after aspartate residues. The first members of this family to be described were the C. elegans gene ced-3 (Yuan et al., 1993) and mammalian interleukin-1β-converting enzyme (ICE) (Cerretti et al., 1992; Thornberry et al., 1992), but many other related genes have recently been cloned from mammals and insects (Alnemri, 1997; Song et al., 1997; Ahmad et al., 1997). One method used by viruses to interfere with the host’s apoptotic response to infection is to disable caspases. For example, the crmA, encoded by cowpox virus, binds with high affinity to ICE (caspase-1) and FLICE (caspase-8) (Ray et al., 1992; Srinivasula et al., 1996), acting as a pseudosubstrate to prevent their proteolytic activity (Ray et al., 1992; Komiyama et al., 1994). In vitro, crmA can also inhibit granzyme B, a serine protease expressed by cytotoxic T cells (Quan et al., 1995). The finding that lymphocytes from transgenic mice expressing crmA were resistant to apoptosis induced by ligation of CD95, but remained sensitive to apoptosis induced in other ways, such as by irradiation or treatment with dexamethasone, showed that apoptosis could occur by independent pathways, only some of which required crmA-inhibitable caspases (Smith et al., 1996). Recently a mammalian gene has been identified that closely resembles crmA. This protein, PI-9, can, like crmA, inhibit granzyme B in vitro (Sun et al., 1996), but its activity with respect to the caspases has not been published to date.

198 C.J.HAWKINS, E.J.COULSON AND D.L.VAUX

p35—A Baculovirus Apoptosis Inhibitory Protein p35 was originally identified as one of a pair of non-overlapping genes from Autographa californica nuclear polyhedrosis virus (AcNPV) which were transcribed in opposite directions from the same region of DNA (Friesen and Miller, 1987). At the time it was cloned its function was unknown; its sequence did not reveal the presence of any recognisable motifs, and it showed no homology to any other gene. The role of p35 was revealed from its recognition as the gene mutated in the “Annihilator” strain of the AcNPV baculovirus (Clem et al., 1991). When susceptible insect cells are infected with Annihilator virus, in contrast to wild type baculovirus, many of the cells die by apoptosis (Hershberger et al., 1992; Clem et al., 1994). DNA encoding p35 was able to inhibit apoptosis caused by Annihilator virus. p35 was also able to prevent insect cell apoptosis triggered in other ways, such as treatment with actinomycin D (Crook et al., 1993; Clem and Miller, 1994). A p35 homologue has been found in one other baculovirus species to date, Bombyx mori nuclear polyhedrosis virus (BmNPV). The BmNPV p35 is very similar at to that from AcNPV, and can also prevent the apoptotic response of insect cells to infection. However, cellular homologues of p35 are yet to be identified. Baculovirus p35 is able to prevent apoptosis in cells from organisms other than insects. Mammalian neuronal cells were protected from death induced by withdrawal of either serum or glucose, or treatment with a calcium ionophore (Rabizadeh et al., 1993). The ability of p35 to prevent neuronal death was confirmed by Martinou et al. using sympathetic neurons dissociated from the superior cervical ganglia of newborn rats (Martinou et al., 1995). In this system cells microinjected with p35 cDNA exhibited a survival advantage over control cells following nerve growth factor withdrawal. Bcl-2 also conferred this antiapoptotic effect, but this could be blocked by coexpression of the Bcl-2 family pro-apoptotic member Bcl-XS. The protection mediated by p35 was not blocked by Bcl-XS, which indicated that p35 worked in a different way to Bcl-2. In mammalian cells p35 was able to suppress apoptosis induced by ligation of Tumor Necrosis Factor Receptor (TNFR) family members such as TNFRI and CD95. This was demonstrated by Beidler et al. using cell lines sensitive to TNF and anti-CD95 antibodies (Beidler et al., 1995). They showed that p35 not only inhibited the morphological changes associated with apoptosis, but also the downstream molecular events such as cleavage of PARP by caspases. p35 can also prevent apoptosis in nematodes, as when expressed in C. elegans, it protected many cells from developmentally programmed apoptosis (Sugimoto et al., 1994). It could also partially rescue the lethality of a ced-9 mutation (Sugimoto et al., 1994), suggesting that, as had been shown for Bcl-2 (Vaux et al., 1992; Hengartner and Horvitz, 1994), p35 works at the same step or downstream of ced-9 in a well conserved apoptotic pathway. In Drosophila, p35 has been shown to prevent developmental cell death (Hay et al., 1994) and that induced by enforced expression of the fly cell death genes rpr (White et al., 1996), hid (Grether et al., 1995) and grim (Chen et al., 1996). The mechanism by which p35 protects cells from apoptosis was determined by Bump et al. (1995) and Xue and Horvitz (1995). p35, like crmA, binds to the caspases and inhibits them from cleaving other substrates. p35 is cleaved as a result of this binding and the two


halves remain attached to the caspase molecule. For this reason, p35 must be present in at least an equimolar concentration with respect to the caspase(s) which it needs to inhibit (Xue and Horvitz, 1995; Bertin et al., 1996). p35 can inhibit a wider range of caspases than crmA, as it has been shown to directly inhibit Caspases 1 (Xue and Horvitz, 1995; Bump et al., 1995), 2 (Bump et al., 1995), 3 (Bump et al., 1995), 4 (Bump et al., 1995) and ced-3 (Xue and Horvitz, 1995) but not granzyme B (Bump et al., 1995). Recently insect caspases from Spodoptera frugiperda; a baculoviral host species (Ahmad et al., 1997) and Drosophila (Song et al., 1997) have been identified, which may represent natural targets of p35. THE IAP FAMILY OF APOPTOSIS INHIBITORS Insect Virus IAP Proteins In an attempt to isolate genes for proteins with similar activities to p35, a modified version of the complementation strategy which yielded p35 was repeated using DNA from a second baculovirus strain, Cydia Pomenella granulosis virus (CpGV), cloned into cosmids (Crook et al., 1993). One cosmid was capable of preventing apoptosis induced by infection with the p35 mutant baculovirus. The gene identified was not related to p35, and was designated IAP, for Inhibitor of Apoptosis (Crook et al., 1993). Unlike p35, the sequence of IAP bore identifiable motifs. It contained a zinc finger of a previously described type referred to as a Ring Finger (Saurin et al., 1996), similar to those found in PML (Kakizuka et al., 1991), BMI-1 (Haupt et al., 1991), and c-cbl (Blake et al., 1991) among many others. It was initially speculated that, like some other zinc fingers, Ring Fingers may bind nucleic acid (Lovering et al., 1993). This, however, has not been confirmed in physiological conditions. Although the crystal structures of the RING finger containing molecules PML and XNF7 (Borden et al., 1995a; Borden et al., 1995b) have been solved, the function of the RING finger domain is still not known. It has been speculated that this domain mediates proteinprotein interactions (Borden et al., 1995a; Borden et al., 1995b), but this has not been experimentally demonstrated. Another possibility is that the RING finger acts as a structural component that does not bind to other molecules, but acts to hold other folds of the polypeptide in place. The RING finger motif found in IAP is slightly different from those found in other proteins, with the consensus sequence being CX2CX11CXHX3CX3CX 7–8CPXCR. In IAP, unlike most other RING finger-containing proteins, this domain is located in the carboxyl half of the protein. The other discernible feature of the IAP sequence was a repeated motif in the amino terminal half of the molecule, which was termed BIR (for baculoviral IAPlike repeat). This motif, with the consensus GX2YX4DX3CX2CX6 WX9HX6–10C, was repeated twice in IAP (Crook et al., 1993). Although the conservation of cysteine and histidine residues in this motif suggested metal binding properties (Birnbaum et al., 1994), the function of the BIRs was unclear at the stage that the baculoviral IAPs were cloned. The sequence of the IAP cloned from CpGV (CpIAP) resembled two stretches of DNA in the Genbank database, one from AcNPV and one from Chilo iridescent virus (CIV). These


genes are designated AcIAP and CiIAP to reflect their sources. A fourth baculovirus IAP gene was isolated from Orygia pseudosugata NPV by complementation of the annihilator phenotype, and is called OpIAP. Of these LAPs, CpIAP and OpIAP can act like p35 to prevent apoptosis, while the less closely related AcIAP is inactive (Birnbaum et al., 1994). It is possible that AcIAP has another function, unrelated to the control of apoptosis, which promotes its retention in the AcNPV virus, although an AcIAP deficient virus displayed a wildtype phenotype. In addition to blocking apoptosis when expressed as part of the virus, CpIAP and OpIAP, like p35, also had anti-apoptotic properties when expressed from plasmids in the absence of the other viral genes (Clem and Miller, 1994). This showed that the IAPs were acting directly on components of the insect cells’ apoptosis machinery, rather than in conjunction with other viral gene products. In experiments assaying the IAPs’ ability to prevent actinomycin D induced cell death, OpIAP gave consistently better protection than CpIAP (Clem and Miller, 1994). As had been shown in the context of the whole virus, AcIAP was unable to prevent apoptosis when expressed from a plasmid, and further was unable to increase the protection conferred by p35 when both were expressed in the same cells (Clem and Miller, 1994). Interestingly, Bcl-2 and a viral homologue E1B-19K were incapable of suppressing apoptosis in this assay, suggesting that IAP and Bcl-2 act in separate pathways. OpIAP, like p35, could also potently prevent apoptosis triggered in the insect cell line SF21 by overexpression of rpr (Vucic et al., 1997), whereas the protection offered by Bcl-2 family members was negligible. The creation of chimeric molecules between the functional CpIAP and the inactive AcIAP allowed the definition of regions which were essential for the protection offered by CpIAP (Clem and Miller, 1994). Regions at both end of CpIAP (both the BIRs and the RING finger) were required for its function, as replacing either region with the corresponding sequence from AcIAP abolished protection. Swapping the RING finger section with that from OpIAP however still produced an active molecule (Clem and Miller, 1994). Protection by IAPs in Evolutionarily Divergent Species To investigate whether the protection conferred by the baculoviral lAPs was restricted to insect cells, its ability to inhibit mammalian cell death was investigated. The most well studied component of the apoptotic machinery are the ICE-like cysteine proteases (caspases). Overexpression of caspases causes apoptosis in many cell types, as first demonstrated by Miura et al. (1993), who transfected cells with plasmids expressing caspases and the β-galactosidase enzyme, and assayed the viability of blue staining cells. Using this technique it was shown the OpIAP could protect against death induced by overexpression of Caspases 1 (Hawkins et al., 1996, Duckett et al., 1996) and 2 (Hawkins et al., 1996), but not 7 (Hawkins et al., 1996). OpIAP was also partially able to suppress death caused by overexpression of a component of the CD95 signalling machinery; FADD/ MORT-1 (Boldin et al., 1995,Chinnaiyan et al., 1995, Hawkins et al., 1996). Using Sindbis virus, it was shown that OpIAP could inhibit apoptosis triggered by viral infection of mammalian cells (Duckett et al., 1996). These results indicated that, like p35 and Bcl-2, IAP proteins could inhibit components of the apoptosis signal transduction machinery which are conserved between evolutionarily divergent species.


a fileref="90000003200073b0f0110b1" id="90000003200073b0f0110b1"/> Figure 7.1 IAP Family Phylogenetic Tree. The full length amino acid sequences (except NAIP; for which only the BIR-containing portion was used) of the IAP family members identified to date were phylogenetically compared. Relationships were determined from a CLUSTAL5 alignment of the amino acid sequences using programs from the PHYLUP program package. The phylogeny shown was calculated by a rooted, distance matrix (PROTPARS, KITCH) on default setting with random input order. Bootstrap analysis using 100 replicas (SEQBOOT, CONCENSE) shows the degree of confidence for the position of each branch to the others. The Genbank assession numbers used for this analysis were L22564 (OpIAP), L05494 (CpIAP), M96361 (AcIAP), M81387 (CiIAP), L49440 (DIAP-1), L49441 (DIAP-2), U19251 (NAIP), U36842 (MIHA), U45880 (xiap), U37547 (MIHB), L49433 (mcIAP-1) U37546 (MIHC), U27466 (ITA).

Cellular IAP Genes As baculoviral IAPs can prevent death of mammalian cells, they must interact with components of the apoptotic machinery that are conserved between insects and mammals. It seemed likely that the IAPs may have cellular homologues in insects (presumably these would be the ancestral source from which the baculoviruses obtained their IAP genes). It was also probable that there would be IAP homologues in mammals, since the insect virus IAPs inhibit mammalian cell death. Such cellular IAP homologues from mammals, insects and birds do in fact exist, and have been cloned independently by several groups. Figure 7.1 illustrates the similarities of the IAPs published to date. Mammalian IAPs The first cellular gene bearing homology to the viral lAPs was one of two candidate genes for Spinal Muscular Atrophy (SMA), a fatal autosomal recessive neuronal disease (Munsat, 1991; Roy et al., 1995). SMA is classified into three types based on severity and age of onset. The disease is characterised by neuronal degeneration, stemming from death of the anterior horn cells of the spinal cord. The gene(s) for SMA had been previously shown to lie in a region of chromosome 5q13. The Survival Motor Neuron (SMN) and Neuronal Apoptosis Inhibitory Protein (NAIP) genes were cloned by mapping the region and determining which genes were deleted or mutated in SMA sufferers but not in unaffected individuals. SMN mutations or deletions were found in almost all patients with SMA. Lesions in the NAIP gene however were less common, and tended to be found in patients with more severe SMA (Roy et al., 1995; Wirth et al., 1995; Burlet et al., 1996; Hahnen et al., 1995). Around 67% of patients with the more serious, early onset type I SMA, also known as Werdnig-Hoffmann disease, had defets in NAIP, compared with 42% for patients with types II or III SMA (Roy et al., 1995). Analysis of the sequence of NAIP revealed the presence of the BIR motif first described in IAP genes. As the SMA phenotype results from inappropriate death of specific neurons (Fidzianska et al., 1990), the demonstration that one of the candidate SMA genes shared homology with IAPs suggested that the wild type NAIP protein may act by inhibiting cell death. Unlike the baculoviral IAPs, NAIP does not possess a RING finger. The remainder of


the NAIP gene contains regions which resemble a GTP/ATP binding site and membrane spanning domains. Three other mammalian IAP homologues were identified in EST databases by virtue of their similarity to baculoviral IAPs (Liston et al., 1996; Uren et al., 1996; Duckett et al., 1996). These genes were given different names by the various laboratories. One, located on the X chromosome was named MIHA (Uren et al., 1996), hILP (Duckett et al., 1996) and xiap (Liston et al., 1996). Other IAPs, closely related to each other were called MIHB/ hiap-1 and MIHC/hiap-2 (Uren et al., 1996; Liston et al., 1996). For simplicity, the genes will be referred to as MIHA, MIHB and MIHC hereafter. Another group cloned two of the mammalian IAPs while searching for proteins which formed a complex with the cytoplasmic domain of TNFRII. TNFRII (p75) is a member the TNFR superfamily which usually triggers cellular proliferation via activation of NF-κB, but can in certain circumstances signal death (Tartaglia et al., 1991; Tartaglia and Goeddel, 1992; Grell et al., 1993). Two TNF receptor associated factors (TRAFI and TRAF2) had already been identified by co-immunoprecipitation, followed by protein sequencing (Rothe et al., 1994). An additional protein was also associated with the TNFRII/TRAF complex, and when purified and sequenced, was found to resemble the baculoviral IAPs (Rothe et al., 1995a). The gene encoding this protein, and a second highly related gene were cloned. They were called c-IAP1 and 2 (Rothe et al., 1995a), and were identical to MIHB and C respectively. Association of IAPs with TRAFs To date the only IAPs that can bind to TRAF molecules are MIHB (c-IAP1) and MIHC (cIAP2) which can both bind to TRAF1 and TRAF2. TRAF2 also binds to the intracellular region of TNFRII. TRAF1 binds to TNFRII indirectly, via its association with TRAF2. MIHB and C were shown to interact, via their BIR motifs, with the C-terminal half of TRAFs 1 and 2. By virtue of their association with the TRAFs, the IAP molecules are recruited to the TNF receptor upon ligand binding (Rothe et al., 1995). In addition to binding IAPs and TNFRII, TRAF2 can also interact with TRADD, and via this association transduces signals for NF-κB and JNK activation from the TNFRI and CD40 (Rothe et al, 1995b; Hsu et al., 1996; Liu et al., 1996). Despite its association with proteins which can transmit apoptotic signals (TNFRII and TRADD), TRAF2 does not appear to play a role in apoptosis signal transduction, as dominant negative truncated TRAF2 proteins that prevent activation of NFκB by TNFR family members do not prevent induction of cell death by TNF (Rothe et al., 1995b; Hsu et al., 1996; Liu et al., 1996). Endogenous insect IAPs Two Drosophila IAP genes have also been cloned. Hay et al. (1995) found DIAP1 in a screen for enhancers of cell death triggered by enforced ectopic expression of the cell death gene reaper in the fly eye. This group and others also cloned a related Drosophila gene (DIAP2/ DIHA/dILP) through searches of gene databases (Hay et al., 1995; Uren et al., 1996; Liston et al., 1996).


An Avian IAP gene A T cell specific chicken IAP homologue has also been published (Digby et al., 1996), but its anti-apoptotic potential has not been reported to date. Protection Conferred by Cellular IAPs Two of the mammalian IAPs (MIHA and B) can inhibit apoptosis mediated by overexpression of Caspase-1, but the protection conferred is not complete, and is not as great as that afforded by OpIAP (Uren et al., 1996). Whe death is induced by overexpression of FADD, OpIAP rescues around half the FADD-sensitive transfected cells (Hawkins et al., 1996) whereas the mammalian proteins gave negligible protection (Uren et al., 1996). Confirming these results, Duckett et al. also showed that MIHA could suppress death induced by Caspase-1 overexpression, and additionally demonstrated protection against apoptosis triggered by Sindbis virus infection (Duckett et al., 1996). Liston et al. described some level of protection by all three MIH genes against apoptosis induced by menadione treatment and serum withdrawal (Liston et al., 1996). The Spinal Muscular Atrophy gene, NAIP, was also capable of inhibiting cell death in these assays (Liston et al., 1996), with its strongest effect being against apoptosis induced by treatment with menadione. Both DIAP1 and 2 could inhibit developmental (Hay et al., 1995), rpr induced (Hay et al., 1995; Vucic et al., 1997) and hid induced Drosophila cell death (Hay et al., 1995). Expression of a truncated form of MIHB in Drosophila eyes could partially alleviate reaper induced apoptosis, although the full length protein did not protect (Hay et al., 1995). Structure of Cellular IAPs The overall structure of these cellular lAPs is similar to the viral proteins, except that all but DIAP1 possess three BIRs, rather than two, and NAIP lacks the RING finger motif. The homology between the IAPs is strongest in the BIR and RING finger domains, suggesting that these are functional motifs. There is controversy in the literature about the functional roles of these domains. The baculoviral lAPs require both the BIRs and RING finger to prevent apoptosis in both insect and mammalian systems (Clem and Miller, 1994; Hawkins et al., 1996 and our unpublished results). However in the Drosophila eye, both DIAP1 and MIHB appeared more effective when their RING finger are removed (Hay et al., 1995). It was surprising that the mammalian IAPs were generally less efficient apoptosis inhibitors than the insect virus OpIAP (Uren et al., 1996), although one group reported equivalent protection from OpIAP as MIHA (Duckett et al., 1996). There are numerous possible explanations for a less potent anti-apoptotic effect of OpIAP than its cellular homologues. Possibly there are factors in the cell lines used which modulate the activity of MIH proteins, but are ineffective against IAPs encoded by insect viruses. It seems plausible that the cells may carry genes which regulate the activity of their own apoptosis inhibitors, and it also is likely that viruses would benefit from mutations in the death inhibitor genes they carry which render their products no longer susceptible to this control by the host.


It is curious that MIHB and MIHC are the most similar of the mammalian lAPs, but MIHB is much more potent that MIHC. The negligible protection offered by MIHC is reminiscent of AcIAP, which is also non-functional (with respect to the assays performed to date). It is possible that both of these proteins work to inhibit other apoptosis pathways than those tested. Alternatively they may have roles not related to the regulation of apoptosis, or they may have once been used to inhibit cell death, but with this role fulfilled by other genes (p35 in the case of AcNPV and perhaps MIHA or MIHB for mammals), there has been no selective pressure against mutations, leaving the products of the genes inactive. The observation that some IAP genes could bind to TRAFs was intriguing, as it suggested a mechanism of action for the IAPs—to inhibit signalling by members of the TNFR family. However three lines of evidence make this theory less plausible. Neither the baculoviral IAPs nor MIHA can bind to the TRAFs assayed to date (Uren et al., 1996), and our unpublished results indicate that the neither MIHA, B nor C can bind TRAFs 4, 5 and 6. Also, TRAF2 dominant negative constructs repress NF-κB and JNK activation, but do not affect apoptosis signalling (Rothe et al., 1995b; Hsu et al., 1996; Liu et al., 1996). Thirdly, and less conclusively, pro teins which interact with IAPs to inhibit cell death would be expected to be found in distantly related animals, as the LAPs are. A search of the Genbank databases revealed that the TRAF-C region of TRAF2 (which interacts with MIHB and MIHC) did not resemble any non-mammalian genes. This would be consistent with TRAFs being an evolutionarily recent development, appearing first during mammalian evolution. Of course, mammalian genes may be over-represented in these databases, so there may be TRAFs from insects, for example, which have not yet been cloned. During evolution IAPs were probably retained to control cell death. Presumably through a series of duplication events, mammals came to possess multiple copies. Possibly in mammals some of these like MIHA still act primarily to inhibit apoptosis, whereas other (like MIHB and C) have evolved other roles. With the evolution of the TRAFs, MIHB and C could have adapted to bind them and thereby influence signalling pathways controlled by TRAFs. Hopefully, gene deletion studies will reveal more of the roles of the cellular IAP homologs. CONCLUSION Valuable lessons about the regulation of cell death have been learnt from studies of baculoviral inhibitors of apoptosis. The elucidation of the mechanism of action of p35 has permitted an appreciation of the importance of the caspases in apoptosis, and will assist in research which could lead to the generation of therapeutic agents to modulate cell death. The search for the mechanism by which IAPs exert their antiapoptotic effects is still underway; hopefully the answer to this problem may be as revealing and useful. The association of mutations in an IAP gene (NAIP) in Spinal Muscular Atrophy raises the possibility of the involvement of the IAP family in neurological and other degenerative diseases, possibly including Motor Neurone Disease (Jackson et al., 1996).


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8. THE MITOCHONDRION: DECISIVE FOR CELL DEATH CONTROL? CATHERINE BRENNER* AND GUIDO KROEMER*† *Centre National de la Recherche Scientifique, Unité Propre de Recherche 420, 19 rue Guy Môquet, F-94801 Villejuif, France

KEY WORDS: apoptosis, mitochondrial transmembrane potential, necrosis, permeability transition, programmed cell death.

INTRODUCTION Apoptosis is a strictly regulated (“programmed”) device which allows the removal of superfluous, aged, or damaged cells. Apoptosis constitutes a physiological mechanism, but its control can become deficient and lead to numerous pathologies. Thus, an abnormal resistance to apoptosis induction generates malformations, autoimmune disease or cancer due to the persistence of superfluous, selfspecific, or mutated cells, respectively. In contrast, elevated apoptotic decay of cells participates in acute diseases (infection by toxinproducing microorganisms, ischemia-reperfusion damage, infarction, apoplexy), as well as in chronic pathologies (neurodegenerative and neuromuscular diseases, AIDS). The process of apoptosis can be subdivided into a least three different phases: initiation, effector, and degradation. During the initiation phase, cells receive apoptosis-triggering stimuli. For example, in mammalian cells, such apoptosistriggering stimuli include numerous toxins, suboptimal culture conditions, interventions on second messenger systems, and ligation of certain receptors (Fas/APO-1/ CD95, TGF-R, TNF-R, etc.) or, in the case of obligate growth factor receptor, the absence of receptor occupancy (Barr and Tomei, 1994; Kroemer, 1995; Kroemer, 1997b; Kroemer et al., 1997a; Kroemer et al., 1995; Kroemer et al., 1997b; Thompson, 1995; Wertz and Hanley, 1996). Non-specific or receptor-mediated death induction involves a stimulus-dependent (“private”) biochemical pathway, and it is only after this initiation phase that common pathways come into action. In spite of the striking heterogeneity of apoptosis induction pathways, some characteristics of the apoptotic process are near-to-constant and do not depend on the induction protocol. It is generally assumed that the execution phase of apoptosis defines the “decision to die” at the “point-of-

† Corresponding Author: 19, rue Guy Môquet, B.P. 8, F-94801 Villejuif, France. Tel.: 33–1–49 58 35 13. Fax: 33–1–49 58 35 09. e-mail: [email protected]

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no-return” of the apoptotic cascade. It is at this level that the different private pathways converge into one (or few) common pathway(s) and that cellular processes (redox potentials, expression levels of oncogene products including Bcl-2 related proteins) have still a decisive regulatory function. During the execution phase, the “central executioner”, “great integrator”, or “apostat” would be activated, thus sealing the cell’s fate. Subsequently, the cell has been irreversible committed to death and the different manifestations of degradation phase become apparent. This degradation phase is similar in all cell types. It is characterized by the action of catabolic enzymes, including specific proteases (caspases) and endonucleases, within the limits of a near-to-intact plasma membrane. Thus, the cell actively contributes to its removal in a “suicidal” fashion and undergoes stereotyped biochemical and ultrastructural alterations. These changes include certain nuclear features of apoptosis such as chromatin condensation and DNA fragmentation, plasma membrane alterations (exposure of phosphatidylserine residues on the outer leaflet), as well as cytoplasmic changes (cell shrinkage, hyperproduction of reactive oxygen species (ROS), activation and action of certain proteases or caspases) (Cohen, 1991; Kroemer, 1995; Kroemer et al., 1995; Thompson, 1995). The discovery that programmed cell death (PCD) may be induced in anucleate cells (cytoplasts) (Jacobson et al., 1994; Nakajima et al., 1995; Schulze-Osthoff et al., 1994) has led to the postulation of a cytoplasmic (non-nuclear) effector or “central executioner” that would participate in life/death decision making and would be influenced by endogenous control mechanisms (Henkart, 1995; Jacobson et al., 1994; Martin and Green, 1995; Oltvai and Korsmeyer, 1994). The exact nature of this cytoplasmic executioner is a matter of debate. In particular, the putative role of mitochondria in the apoptotic effector stage now emerges more clearly from studies based on cell-free systems and molecular functional analysis. The present review will summarize evidence indicating that mitochondria play a major role in the apoptotic effector phase. NO NEED FOR MITOCHONDRIA IN APOPTOSIS? Cells lacking mitochondria DNA can undergo apoptosis The mitochondrial genome of human cells encodes RNA molecules necessary for mitochondrial protein synthesis (12 S and 16S ribosomal RNAs, 22 different transfer RNAs) and a few subunits of the multiprotein respiratory chain complex I (6 NADH dehydrogenase subunits), complex III (apocytochrome b), complex IV (cytochrome b oxidase subunits 1, 2 and 3), and the ATP synthase subunits 6 and 8 (Borst et al., 1984; Gray, 1989). However, most mitochondrial proteins are synthesized in the cytosol as precursors and are selectively addressed to either of the two mitochondrial membranes, the intermembrane space or the matrix (Schatz and Dobberstein, 1996). Thus, mitochondria lacking mtDNA have a near-tonormal morphological aspect and can divide by fission to replicate indefinitely in the cytoplasm of the proliferating cell. With respect to the status of their mtDNA, cells can be normal (ρ+), carry deletions of part of the mitochondrial genome (ρ–mutants) or may have deleted the entire mitochondrial genome (ρ°). The rationale for the isolation of human ρ° cells is based on the use of

212 C.BRENNER AND G.KROEMER

inhibitors of mtDNA replication, such as the DNA intercalating dye ethidium bromide, which, at low concentrations (0.1 to 2 µg/ml), demonstrates a relatively selective incorporation into the mitochondrial vs nuclear DNA, resulting in either partial or complete inhibition of mtDNA replication without affecting nuclear DNA synthesis (King and Attardi, 1996; Marchetti et al., 1996d). It may be important to note that ρ° cells require relatively large amounts of glucose since they cannot generate ATP via oxidative phosphorylation and entirely rely on glycolytic ATP generation. Moreover, ρ° cells are generally auxotrophic for uridine and pyruvate. Failure to provide such metabolites implies that ethidium bromide becomes toxic for cells. At least part of this toxicity involves induction of apoptosis (Baixeras et al., 1994). Although the electron transport on the inner mitochondrial membrane is defective in cell lacking mitochondrial DNA, ρ° cells possess a normal mitochondrial transmembrane potential (∆Ψm) (Marchetti et al., 1996c; Skowronek et al., 1992) and fulfill a number of metabolic functions including import of proteins encoded by nuclear genes. It is unclear how the ∆Ψm which is essentially a proton gradient is generated in ρ° cells. Irrespective of the exact mechanism of ∆Ψm generation, it appears that inhibition of glycolysis by NaF causes an immediate disruption of the ∆Ψm (Skowronek et al., 1992). Thus, maintenance of the ∆Ψm in ρ° cells requires anaerobic glycolysis. The finding that ρ° cells can readily undergo apoptosis has been first reported by Martin Raff’s group (Jacobson et al., 1993) and then confirmed in numerous studies. Thus, human GM701 ρ° fibroblasts die from apoptosis in response to antiCD95 and staurosporine, a protein kinase inhibitor (Jacobson et al., 1993). Similarly, U937 ρ° cells rapidly undergo apoptosis upon stimulation with TNF or anti-CD95 (Gamen et al., 1995; Marchetti et al., 1996c). Normal ρ+ cells exposed to inhibitors of the respiratory chain die either from necrosis or apoptosis, depending on the cell type (Shimizu et al., 1995; Wolvetang et al., 1994). As to be expected, ρ° cells (which lack a functional respiratory chain) are completely resistant to the apoptosis-inducing effect of respiratory chain inhibitors such as rotenone and antimycin A (Marchetti et al., 1996c; Wolvetang et al., 1994). Moreover, they are resistant to the apoptosis-inducing effect of hyperoxia (Yoneda et al., 1995), presumably because they fail to generate toxic oxygen radicals on the respiratory chain. The above observations indicate that mtDNA-dependent cellular functions (mainly oxidative phosphorylation and mitochondrial respiration) are not required for the common pathway of apoptosis. Although this interpretation has been clearly formulated in this cautionary form (Jacobson et al., 1993), many investigators overinterpreted the fact that ρ° cells can undergo apoptosis to mean that mitochondria as such are not involved in the apoptotic process. This notion was apparently underscored by the finding that the electron microscopic picture of mitochondria remains essentially normal until late stages of the apoptotic degradation phase. Moreover, in contrast to nuclear DNA, mitochondrial DNA is not degraded during apoptosis (Tepper and Studzinski, 1992; Tepper and Studzinski, 1993), although it may be fragmented during necrosis (Tepper and Studzinski, 1993). Nonetheless, the experiments performed on ρ° cells do not exclude the participation of mitochondria in apoptosis. It would be necessary to produce cells without mitochondria rather than cells without mitochondrial DNA, and such cells are not viable. The putative involvement of mitochondria in the death process, prompted us to study the mitochondrial function in U937 ρ° cells which undergo apoptosis in response to TNF (Marchetti et al., 1996c). We had previously described that a ∆Ψm disruption was observed


early during the apoptotic precess in numerous p+ cells. This ∆Ψm dissipation is meadiated by opening of so called permeability transition (PT) pores (Kroemer, 1997a; Kroemer, 1997b; Kroemer et al., 1997a; Kroemer et al., 1995; Kroemer et al., 1997b; Petit et al., 1996). Importantly enough, we found that ρ° cells manifest an early disruption of the ∆Ψm, exactly as this is observed in TNF-treated ρ+ controls (Marchetti et al., 1996c). In addition, we showed that isolated mitochondria from ρ° cells undergo PT, a phenomenon that we have implicated in apoptosis, in response to a variety of stimuli (atractyloside, calcium, terbutylhydroperoxide) in a fashion that is indistinguishable from control mitochondria (Marchetti et al., 1996c; Zamzami et al., 1996b). This in line with the fact that all putative components of the PT pore are encoded by nuclear rather than mitochondrial genes: the adenine nucleotide translocase, the peripheral benzodiazepine receptor, and the voltagedependent anion channel (Bernardi et al., 1994; Brandolin et al., 1993; Zoratti and Szabo, 1995). Moreover, ρ° mitochondria liberate the same amount of apoptogenic factors from the intermembrane space as do ρ+ control organelles (Marchetti et al., 1996c; Zamzami et al., 1996b). Such apoptogenic factors are capable of causing isolated nuclei to undergo hallmarks of nuclear apoptosis (chromatin condensation and DNA fragmentation). Their presence in the supernatant of ρ° mitochondria indicates that they are encoded by nuclear rather than mitochondrial genes. In conclusion, it appears that the structures responsible for PT and its regulation, as well as the mitochondrial proteins with apoptogenic properties, function normally in ρ° cells. Therfore, the fact that ρ° cells can undergo apoptosis normally cannot be used as an argument against the involvement of mitochondria in apoptosis. Cell-free system apparently not implying mitochondria Cell-free systems, that are in vitrocultures of disassembled cellular components, have been designed to recapitulate essential steps of the apoptotic process in vitro, for instance by combining isolated nuclei with cytosolic extracts and/or isolated organelles, to determine the conditions in which chromatin condensation and endonuclease-mediated DNA fragmentation will occur. A number of cell-free systems involved the use of cytosolic extracts from chicken mitotic cells (Lazebnik et al., 1993; Lazebnik et al., 1995b), from Xenopus laevis oocytes undergoing follicular atresia (Cosulich et al., 1996; Evans et al., 1997; Newmeyer et al., 1994), or from various human and murine transformed cell lines including U3937, BT-20, HL-60 and 3T3 cells (Enari et al., 1995; Enm et al., 1996; Martin et al., 1995; Shimizu and Pommier, 1996; Wright et al., 1994; Yoshida et al., 1996). Such cytosolic extacts are generated by mechanical or detergent-mediated disruption of cells and cycles of freeze-thawing, followed by ultracentrifugation. Thus, apparently they are organellefree. They can be employed to induce key features of nuclear apoptosis in vitro: chromatin condensation, internucleosomal DNA fragmentation, or cleavage of nuclear caspase substrates including PARP and lamins. The fact that such extracts are organelle-free and that addition of caspases to extracts from normal cells creates an apoptogenic activity (Enari et al., 1995; Enari et al., 1996; Muzio et al., 1997), has been thought to imply that mitochondria would not be involved in apoptosis and that rather soluble factors including caspases would be decisive for the cell death process.

214 C.BRENNER AND G.KROEMER

This interpretation can be criticized at two levels. First, it has to be critically noted that cell-free systems by definition involve a disruption of subcellular compartimentation, and then, do not necessarly reveal the true physiology of the system. Thus, it can not be excluded that components are relased from various organites (mitochondria, lysosomes, endoplasmic reticulum) during the preparation of the cytosolic extracts. Second, it may be questioned as to whether such cell-free systems truely reveal the execution phase of apoptosis. Indeed, the nuclear degradation, which is the principal read-out of cell-free systems of apoptosis, forms part of the degradation phase and even is not necessary for cell death to occur (Jacobson et al., 1994; Schulze-Osthoff et al., 1994). Cell-free systems based on the use of carefull separated subcellular fractions have recently suggested that mitochondria and/or mitochondrial products are required for the induction of nuclear apoptosis (Liu et al., 1996; Marchetti et al., 1996c; Newmeyer et al., 1994; Susm et al., 1996; Zamzami et al., 1996b).These data will be discussed in detail in the following section. MITOCHONDRIAL IMPLICATION IN APOPTOSIS: EVIDENCE FROM CELL-FREE SYSTEMS A two-step model of apoptosis: an initial mitochondrial step followed by a secondary nuclear step As discussed above, cytosols from cells undergoing apoptosis contain apoptogenic proteins capable of provoking isolated nuclei to undergo chromatin condensation and DNA fragmentation (Lazebnik et al., 1995a). A similar apoptogenic activity is encountered in whole cytoplasmic preparations of normal cells, and this activity is associated with mitochondria (Martin et al., 1995; Newmeyer et al., 1994). Current data are compatible with a two-step-model of apoptosis. At a first step, different effectors act on mitochondria to disrupt the mitochondrial membrane integrity at the level of the inner and/or outer membrane. As a consequence, mitochondria then release apoptogenic factors into the cytosol, when then can act directly or indirectly on nuclei. This model integrates apparently contradictory data from the litterature. It also predicts that the decisive event of the apoptotic cascade (that is the effector stage) would be disruption of mitochondrial membrane structure or function rather than nuclear demise (that is the degradation stage). What is then occurring during the effector stage of apoptosis? As mentioned above, we have found that early during apoptosis occurring in intact cells, the mitochondrial inner transmembrane potential (∆Ψm) is disrupted and that this ∆Ψm dissipation involves opening of the mitochondrial megachannel, also called the permeability transition (PT) pore. To investigate the relationship between PT pore opening and release of mitochondrial apoptogenic factors, we incubated isolated mitochondria with agents specifically acting on components of the PT pore. Such agents, which include protoporphyrin IX (a ligand of the peripheral benzodiazepin receptor) or atractyloside (a ligand of the adenine nucleotide translocator), cause an immediate release of apoptogenic factors into the mitochondrial supernatant (Marchetti et al., 1996b; Marchetti et al., 1996c; Zamzami et al., 1996b). Such factors include “apoptosis-inducing factor” (AIF) (Susin et al., 1996) and cytochrome c


(Kantrow and Piantadosi, 1997) (see below). In a further attempt to understant the apoptotic process, we sat out to identify physiologically relevant inducers of PT accumulating in the cytosol of cells undergoing apoptosis. It appears that a number of physiological effectors (Ca2+, reactive oxygen species, and nitric oxide), caspases, and additional yet non-identified second messengers elicted by ceramide can cause PT in isolated mitochondria, thereby triggering ∆Ψm disruption and release of apoptogenic factors (Hortelano et al., 1997; Susin et al., 1997a; Susin et al., 1996; Susin et al., 1997b; Zamzami et al, 1996b). In contrast, to these observations, Newmeyer and co-workers suggest the presence of a yet un-identified cytosolic factor that causes the release of cytochrome c from mitochondria without that disruption of the ∆Ψm occurs (Kluck et al., 1997). The exact relationship between PT, release of apoptogenic factors, and ∆Ψm disruption is currently a field of controversy and intense research efforts. Apoptogenic factors released by mitochondria:cytochrome c and AIF Supernatants from mitochondria that have undergone PT but not those from control mitochondria induce signs of apoptosis such as nuclear chromatin condensation in a cell-free system (Susin et al., 1996; Zamzami et al., 1996b). Destruction of mitochondrial membranes via sonication, osmotic shock, or digitonin treatment (which specifically lyses the outer but not the inner membrane, yielding the intermembrane fraction of proteins) also releases this activity, that we have baptized “apoptosis-inducing factor” (AIF), indicating that AIF is pre-formed (Susin et al., 1996) (Table 8.1). Proteinase K treatment as well as heat treatment (70°C, 5 min) destroy AIF activity, indicating that AIF is a protein. Anion exchange chromatography, molecular sieve chromatography, and SDS-PAGE identify mouse hepatocyte AIF as a single ~50 kDa proten. AIF is low-abundant (

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